In my less active days I was an expert on sedentary dispositions.
Oooh, I found a nice picture of diatoms.
Reptiles and amphibians of North America, plus a pretty good knowledge of poison dart frogs and poisonous snakes world-wide
Electronic payments(the US ACH system and online bill payment)
I’m pretty knowledgeable about guitars, guitar repair and guitar playing, but there are others here who know way more than me.
VC++ Version 6
Robotics and automation programming.
I also give a great backrub.
Criminal law
Computer security
Pop music, 1955-1985
Broadway musicals
Opera
Roman Catholicism
The oeuvre of Joss Whedon
And as a bonus:
Pokemon (vicariously, through extensive consultation w/ Bricker Jr. Not as agood as first-hand knowledge, but you can’t ignore a resource like that!)
Product management
Regulatory affairs, primarily Medicare, but also COBRA and ERISA
Archaeology
Linguistics
Languages
Feeling guilty about all the things I should be doing haven’t gotten around to (most of this is related to clutter and reading women’s magazines way too much - I gotta work on the former and stop reading the latter)
I’m not sure most of the posters are defining “Expertise” the way I would. With due respect - this is just a semantics thing - I don’t think very many people are “Experts” at more than a couple of things, and certainly not five or six.
I am an expert at exactly one thing: quality management systems auditing. In that area I am an expert beyond any doubt or question.
I know a lot about other stuff. I am very knowledgeable about baseball, for instance. But I would not say I am an expert at it. I would think that an expert in baseball would be either an actual baseball professional, such as a player or a professional coach, or would be someone whose knowledge and understanding of the theory or history of baseball involved doing original work in expanding the world’s understanding of baseball. I’m a baseball fanatic because I know a lot about the Negro Leagues; a baseball expert is searching archives and finding primary resources doing RESEARCH on the Negro Leagues. I’m a baseball fanatic because I play the game; an expert is someone who gets paid to play the game. I’m a baseball fanatic because I like long, detailed discussions about how the Blue Jays should be run; a baseball expert is someone who actually is part of the Blue Jays staff.
I guess I’m just being picky, but to my mind a thread that talks about what you’re really an expert in would be just a bit more interesting than “what stuff do you know a lot about?” But that’s just me.
Image generator systems and geodatabases for simulation and training.
PC repair, and especially malware removal (I’ve successfully removed viruses that the recommended solution was to wipe the HD and reinstall Windows).
Fitness and training, as it pertains to endurance sports.
Mystery Science Theater 3000
Motorcycles
I’m also a pretty good mechanic and can fix most anything, though I’ll admit there are better.
Drive bys
Which of those did you like best?
In the absence of other information, I’d start you on the Palliser novels, which are, in order, Can You Forgive Her?, Phineas Finn, The Eustace Diamonds, Phineas Redux, The Prime Minister, and The Duke’s Children. These follow the political career of Plantagenet Palliser, who you saw as a minor character in book 5 of the Barchester Chronicles, The Little House At Allington. They are not as comical as the Barchester series, but they definitely have their moments. If you liked the legal stuff in The Last Chronicle of Barset, try Orley Farm or Mr. Scarborough’s Family.
Beer
Booze
Buffy
I see what you’re saying. Though for some people whose hobby is X, if they’re really devoted, they may well be considered an expert, knowing more than most people, even some professionals in their area of interest.
As someone who listed several areas of expertise, mostly it’s because I currently work in product management, am fairly well recognized in my industry as a subject matter expert in Medicare, COBRA and ERSIA and how they work together and have graduate degrees in linguistics and archaeology (which I no longer use, though). Perhaps other dopers are in the same boat - have degrees in areas in which they no longer work, yet are considered experts in their current industries.
As far as guilt goes, I think I have a PhD in guilt - I earned it at the University of Mom. 
Middle Eastern politics.
The Balkans in general.
Human trafficking in the Philippines.
Cheesy genre TV shows.
In decreasing order of expertise, and currently professionally doing the first two:
Linux administration, Windows administration, and the horrific Lovecraftian experience of both simultaneously
Cisco firewalls
Modern military history, strategy, and tactics
Modern science fiction
Video game history, trivia, and mechanics
I won’t claim outright expertise, but rather areas of professional and/or personal knowledge on a level such that I would feel comfortable discussing them with an expert.
Law, particulalry criminal law and procedure, constitutional law, and federal jurisdiction.
Criminal and forensic psychology, particularly insanity and competence, risk assessment, psychology of the serial sex offender, and psychopathy.
Martial arts and combat sports, particularly boxing, kickboxing, mixed martial arts, judo, combatives, and a smattering of stick and blade.
Things I’m fairly knowledgeable about? Jewish theology, zoology, to an extent science in general.
Expert in the Cafe Society sense? Terry Pratchett, backwards and fowards. Also Order of the Stick, and fantasy in general.
Things I tend to do on the board? Trying to get people to get along, I guess.
Expert:
Medieval cooking (mostly 14th - 16th c. timeframe)
Medical billing, and by extention medical insurance and how it works.
Knowledgable beyond normal limits:
SCA Culture
Heraldry
Fandom (mostly SF/Fantasy)
Running a fan-run convention
I know a lot about several TV shows, but I’m not up to true geekery on them: Big Love, Babylon 5, Criminal Minds, NCIS, CSI: Vegas.
That’s all I can think of off-hand.
That is so cool! (It doesn’t take much to get me excited these days.)
We need a process on spinning off the expertise thread…I;m really interested to know your opinion on the medical insurance industry (Is it really as fucked up as it appears from the standpoint of a poor schmuck trying to get treated?)
Can and frequently do delve into geeky minutiae about…
Soccer
Tennis
Martial arts
Kung fu movies
The works of Frank Herbert
Video games
Computers
Dogs
Why everyone who thinks Deckard is a replicant are sad, misguided, befuddled simpletons.