Not as important as others that have been mentioned, but I’ll go with:
“All only children are spoiled.”
(or, “Only children grow up not learning how to socialize.”)
“All stereotypes are necessarily false” 
All small dogs are hideous yappy vermin who deserve to die a painful death.
People who collect things are not socially maladjusted, obsessive-compulsive dorks.
Not all of us, anyway.
That all furries are perverts.
I can’t believe it even shows up here. Precisely why is liking books with anthro animals any more abhorrent than liking Star Trek? I’d think the proportion of freaks in either fandom would be the same.
And that Florida in particular is full of looneys and idiots.
That ugly = evil (I understand this was big in Victoriana, and has a touch of Calvinism to it, but it’s really, really pervasive, especially in fairytales and childrens’ literature).
That gamers (I’m talking pen-and-paper RPGers) are all socially inept. I find this one to end up being a self-fullfilling prophecy in many cases.
Just out of curiousity, I thought that ‘furries’ didn’t refer to the fandom, per se, but to those people who live in the fur suits? Which is a level of dedication to their fandom that seems a bit noteworthy, even to this otaku. Is the word used for both, when talking about people, rather than the comics/stories?
I’d like to see reliance on stereotypical behavior by people in positions of power go away. A recognition that variations within a population make individual predictions based on the population profile suspect. The stereotypes I’m most familiar with (Jewish grandmothers and sailors) have too much truth in them for me to say they’re completely inaccurate.
Wait, so it’s true what they say about Jewish grandmothers and sailors? And here I thought all those letters were fake!
(muttering) :smack: (muttering some more)
I even knew that if I didn’t write that carefully, I’d get this.
And I still forgot to properly proofread.
I’d really liker to end this one stereotype perpetuated by American TV – sitcoms in partucluar:
Hard-working white husbands/fathers are total idiots/dopes that are dominated by their never-failing, always-right wife.
Stereotypes save a lot of time! 
(where the heck did I see that? And why did it stick in my head?)
Substitute “nagging and shrewish” for “never-failing” and you’ve got it right.
I’m quite tired of the advertising stereotype that every woman’s ultimate goal is a huge diamond, fairy-tale wedding and tons of baybees.
That all women are emotional beings who can’t be counted on to think rationally in crisis situations.
And for that matter, the one that says any woman who doesn’t like babies and children is somehow damaged or unhealthy.
Furry refers to the fandom, media with anthro animals, the anthro animals themselves, and the fans. Doesn’t make it easy, does it?
I don’t know that anyone lives in fursuits. I’ve never worn one, but the most advanced ones are like mascot suits - they’re hot, uncomfortable, and expensive. There are some furries that will wear a tail or ears all the time. They generally can be referred to as ‘lifestylers’ and generally are closer to otherkin and getting on the crazy side.
So, there’s furry fans who like the stuff; furry lifestylers who think they’re related to, mentally are, or used to be animals of various anthro-states; and just plain perverts.
Which, to use anime as an example, breaks down the same as otaku, otakukin (except lifestylers & otherkin are slightly less nuts), and perverts.
(FTR, I’m just a furry fan and not a very dedicated one. I lurked on various newsgroups for a couple of years. Most of the artists I liked have since disappeared.)
Really, I’d just like people to stick to the rule that you can’t make fun of groups you aren’t part of or used to be part of.
That just because someone is homeless doesn’t mean they are stupid.
That jo mama isn’t THAT fat…
Since everyone mentioned the big ones already…
That Public Defenders are the lawyers who weren’t good enough to get a “real” job.
The one that says all big dogs are nasty, vicious creatures who enjoy eating children’s faces.
Also, that all SUV drivers are soccer moms or guys compensating for undersized things.
That nurses are dim, submissive women who don’t do much but empty bedpans and clean up after invalids and old people.
(Yes, I still find this attitude, especially among young people, with frightening frequency. And yes, I know some LPNs do this stuff, but it mostly falls to techs and aides. And none of them had to go get a Bachelor’s degree for the privilege of doing it. And–dude, some nurses are…MEN!!!)