I’ve got a little theory I want to test, so I’m starting a poll of sorts. Since I don’t want to risk skewing the results, I’m not telling you what the theory is just yet.
1.) Tell me a little about yourself.
2.) How do you typically address mail to a married couple?
3.) How do you put your return address on mail?
4.) What are your age, gender, and marital status?
1)/4) 24, ex-student, male, single, currently long-term illness, planning on returning to studies for PhD, generally middle-of-the-road middle-class.
If I know their first names, then “Fred and Anne Smith”. Otherwise, Mr and Mrs. I don’t stand on ceremony. And you’ll hardly ever catch me using ‘Dr’.
Return addresses aren’t the norm in the UK, but when I do (ie when writing abroad), I give my full name and address.
1.) I perform arcane, mildly geeky tasks for work. I am an expatriate. I kinda like cats but I’m not crazy about them. I write and paint. (Write well, paint not so much.)
2.) How they prefer that it be addressed; running the gamut from “MilitaryTitle and Mrs. Hisfirstname His Lastname” to “Herfirstname Herlastname and Hisfirstname Hislastname.” I take the cue of how they do their return address or ask a family member.
3.) Myfirstname Mylastname. And his, if we’re addressing it jointly.
4.) 28, F, Married
1.) Tell me a little about yourself.
I’m a 5’5" Virgo who likes stargazing and quiet nights at home. 
Or, alternately, I’m a senior in college, live in WV, and I have a brilliant kid. And I’m a book junkie.
2.) How do you typically address mail to a married couple?
Never have, really. I guess I would write Bob & Jane OscarMeyer-Weiner, though.
3.) How do you put your return address on mail?
Is it just me writing to someone? Then Abbie Carmichael. If it’s from me and the husband, it’s Dick & Abbie Carmichael.
4.) What are your age, gender, and marital status?
26, female, married.
1.) Tell me a little about yourself.
I’m a medical transcription quality assurance specialist originally from Iowa, now living in Minnesota. I live in an apartment with my two incredibly adorable cats. My hobbies and interests include candle making, singing, some other miscellaneous stuff, and I read a great deal.
2.) How do you typically address mail to a married couple?
However they prefer to be addressed. Usually “John and Jane Doe.”
3.) How do you put your return address on mail?
First initial, last name, then address.
4.) What are your age, gender, and marital status?
26, female, single.
This is a pretty broad question! Ummm, me? I’m a marketing manager in a financial services company. When not at work, I’m usually horseback riding (I do hunters), reading, or playing with my two dogs. Had a steady boyfriend, but we broke up not too long ago. I spend way too much time on this message board (usually lurking).
For a formal things: depending on what they prefer (usually whether the woman has kept her last name), either
Mr. and Mrs. Man’s-First-Name Couple’s-Last-Name, or
Mr. Man’s-First- Name Man’s-Last-Name and Ms. Woman’s-First-Name Woman’s-Last-Name.
I do sometimes use “Dr.” or military title as appropriate if the person regularly uses it.
For informal things: again depending on what they prefer, either
I make exceptions if I know the someone prefers something different. For example, some women prefer to be addressed as “Mrs. Ann Smith” (with Smith the couple’s last name) rather than “Mrs. Dan Smith.”
Last-Name
Street Address
City, State Zip
I don’t use my first initial or name.
37, female, single.
**1.) Tell me a little about yourself. **
I love fireplaces, looking for sea glass, koto with clarinet, snow, intense color, Cezanne’s Apples and Oranges, the well-crafted word, well-timed irreverence.
2.) How do you typically address mail to a married couple?
To those of the pre-boomer generations and on formal occasions, I use “Mr. and Mrs.” On all other mail I use first and last names.
3.) How do you put your return address on mail?
Just my last name and then my address
4.) What are your age, gender, and marital status?
I’m 60 and a half, female and married.
I am middle-class, well-educated, raised in the south. Female. 43 (until next month!). Married for 2 years. Politically liberal.
Mary and John Jones (or John and Mary Jones) I have noticed the order usually depends on which one I knew first–their name comes first. Never Mr. and Mrs. And if they have kids, sometimes I use “The LastName Family” especially if I am sending gifts.
With a sticker. Oh, you wanna know what it says? Rick and Brynda LastName
See above.
I’ve thought of a new description for my place of employment " intellectual sweatshop," which I feel is appropriate given quotas and the fact that it’s situated in an old mill building with no ventilation (but there’s going to be a new building in November). I spend way too much time reading and writing X-Files fan fiction…but it’s good, dammit! I find it irritating that dems have tried to steal the term “leftie” from left-handed people.
First name & first name, Surname. Which order I put the names is dependent on which of the couple I know- that name goes first. If I know both, whichever sounds better.
On bills, first initial & last name plus address, but if it’s personal mail I use my full name.
27, female, single.
33-year-old computer programmer living in the SF Bay area. If you want more, I’d say: big, hairy, and nerdy.
Depends on how well I know the couple. If they’re close friends, I say:
(First name of Person I knew first) AND (First name of spouse) LAST NAME
ADDRESS
If I met them both at the same time, I say:
(First name of man) AND (First name of spouse) LAST NAME
ADDRESS
If I don’t know them that well, or if I only know the first name of one of them:
MR AND MRS (First name of man) LAST NAME
MY LAST NAME
STREET ADDRESS
CITY, STATE, ZIP
33, male, legally not permitted to marry
1.) College grad, currently studying and working in biomedical lab technology, living with my mom in Montreal. Bookworm, oldest of 3 kids. Brunette. I look very nice in dark green. 
2.) Mr. and Mrs. LastName, unless I know they prefer otherwise, like if she’s kept her maiden name or something. If they’re good friends, probably “Bob and Jane LastName”.
3.) Never my name. Always just my address.
4.) 23, female, single
I’m Aquarius, a bar manager and bartender, a “TomBoy” (or so I’m told) and a baseball fanatic.
Generally, “Mr. and Mrs. Lastname”. If I know they have kids, and it’s something like a Christmas card, I’ll add “and family.”
If the woman has kept her maiden name then I’ll address it “Mr LastName and Mrs. LastName.”
First initial Last name and address. I usually use just initials for Las Cruces (LCNM), since I figure they’ll know where I am by the zip code.
31, Female, Single
1.) **Tell me a little about yourself. **
Liberal, bisexual atheist Originally from Tennessee but now living in Oregon.
2.) How do you typically address mail to a married couple?
John and Jane Doe; 123 Main St; City, State 98734
3.) How do you put your return address on mail?
Robert Aesiron; 321 Main St; City, State 98734
4.) What are your age, gender, and marital status?
22 year old single white male
37, F, M
1.) Tell me a little about yourself.
I have long brown/red hair and brown eyes. I am 5 ft 2in tall. My face is roundish to heart-shaped. Most folks would consider my body type buxom. I have very faint freckles across the bridge of my nose. I consider myself “cute”, rather than beautiful. I’m dependable and creative. I tend to favor a leadership position in most situations, but am woman enough to admit that it’s because I am slightly bossy. I am a college educated professional, I vote liberal, and I am pagan.
2.) How do you typically address mail to a married couple?
John and Susan Doe
1314 Nowhere St.
SomeTown, XY 101010
3.) How do you put your return address on mail?
With my first initial, last name…and then street, state, zip
4.) What are your age, gender, and marital status?
33, female, happily married
FB
(1.) Tell me a little about yourself.
Overeducated, not living up to my potential, but happy at what I’m doing (I’m an editor and a novelist). I love summer and hate winter. I don’t like Christmas or ice cream. I’m a good dancer. I wish I were more spontaneous and I once asked some people–facetiously–whether there was some kind of class I could take to remedy this (I thought that would be funny. I meant it to be ironic.) Turns out there is some kind of class you can take. Improv. But I haven’t taken it yet.
2.) How do you typically address mail to a married couple?
I don’t actually do this very often but when I do, it’s [one I’m most friendly with] and {his/her spouse} [lastname]. This would likely be a Christmas card, which I send even though I hate Christmas.
3.) How do you put your return address on mail? Last name, address; or just the address (no apt. #, it’s a house)
4.) What are your age, gender, and marital status?
53, F, M
1.) Tell me a little about yourself.
I’m a sexual misfit, another gender dysphoriac of sorts; I’m a diagnosed schizophrenic in the movement against psychiatric oppression and forced treatment; a piano player and composer, Mac user, FileMaker geek, anarchist, and of course Straight Doper. I’m a moderately tall & skinny retrohippie-looking dude with glasses.
2.) How do you typically address mail to a married couple?
Daddy and Mama. (They’re the only married couple I ever write letters to)
3.) How do you put your return address on mail?
Upper left hand corner. Name ¶ # and Street ¶ City State ZIP. If this has something to do with titles like “Mr” and “Ms”, no, I don’t use that stuff.
4.) What are your age, gender, and marital status?
I’m a 45 year old male. I don’t do marriage.
**1.) Tell me a little about yourself. **
I’m a geek, and pretty happy about it. I’m married to Dangerosa and have two kids. I work too much as a manager for the e-commerce side of a major retailer. I play a lot of computer games.
2.) How do you typically address mail to a married couple?
Depends… if they share a last name, then either John and Sue Lastname, if I know him better, or Sue and John Lastname, if I know her better.
If they don’t share a last name then I follow the same rule about who comes first (the person I know better) but include both last names. I never use “Mr. and Mrs.” but have been known to use Mr. John Lastname and Ms. Sue Otherlastname if it’s something formal like a wedding invitation.
3.) How do you put your return address on mail?
If it’s from me then:
Mylastname
123 Street, City 12345
If it’s from our household then:
Mylastname/Herlastname
123 Street, City 12345
(yes, I do put the slash between our last names)
4.) What are your age, gender, and marital status?
38, male, married.
I suspect I am not confirming your theory.
1.) Tell me a little about yourself.
Overeducated, middle-class, works too hard for not enough pay in a job I love and am ideally suited for (puzzle editor)
2.) How do you typically address mail to a married couple?
John and Jane Doe
3.) How do you put your return address on mail?
By hand, last name only – on address labels, Firstname MI Lastname
4.) What are your age, gender, and marital status?
49, hetero female, never married
1.) Tell me a little about yourself.
Sr in college. Currently work as a tutor in the University tutoring center. Own two cats. Hobbies including reading and writing. Currently addicted to the band Wilco.
2.) How do you typically address mail to a married couple?
I can say in all honesty it’s never come up. I guess though, thinking about it, I’d probably use Mr and Mrs last name if Mrs. went by his last name. However, nearly all the couples I know have different last names, so probably just “Bailey and Dave” no last name.
3.) How do you put your return address on mail?
(My last name), Haley. I never put Jaime’s name or last name on the return address…
4.) What are your age, gender, and marital status?
21. Female. Married.