I’m not an expert at anything but I’m pretty knowledgeable when it comes to the drilling of Oil and Gas wells and from there I’m capable of answering most questions about oil and gas development and occasionally I’ll jump in and answer general industry questions. I like geology and I spend a lot of time with geologists but generally a true expert will answer before I can chime in.
Like I said, thirteen years as a professional student. Eventually you start sounding like an Ivory-Tower academic, even if you aren’t one ;).
I am a solid long term winning poker player, although I would never want to play professionally. I am always happy to opine on those issues.
I am also a certified financial planner and can answer most personal finance questions, although many of those answers are necessarily subjective.
I find myself answering viticulture and enology questions the most, and things related to that industry. Packaging manufacturing - corks, glass, capsules, etc. And general alcohol law in the US, TTB regulations, wine label requirements, stuff like that. They don’t come up often in GQ though, usually found more in Cafe Society.
So what do you do (if you don’t mind me asking)?
That could apply equally to the poker stuff too. 
Well, unless you’re talking about pot odds and such, but people tend not to ask those questions.
I’m in the thrilling field of water recycling/reclamation. Journeyman operator - think Homer Simpson, only with more hair and a slightly bigger vocabulary :D.
Can’t complain, really. I sort of fell into the job while I was in school and after awhile you get institutionalized. For a few years after I was hired and was continuing as a student on the side, I fancifully entertained notions of at the very least finishing up on the biology side ( I was a double in history and biology ), picking up my graduate degree and then moving on. But I was never the most disciplined of people and forced into a part-time situation at the university that was subsidized by my job, it became much easier to take fun classes, rather than concrete graduation requirements.
And after several years the steady pay and the siren song of the pension plan in particular becomes too strong to break away. It’s essentially a skilled trade ( I’m licensed by the state by exam and I get paid the same as our electricians ), so the money in my very expensive area is quite good and where I work it is very low stress and independent ( I’ve spent the last month working six days a week by myself and haven’t seen my superior once ). For a low ambition, introverted type like myself, it’s really a near-perfect fit in some senses.
But if I had it to do over, I might have buckled down on the history side early on, picked up graduate degree in library science and gotten a job at some academic institution somewhere. Probably fit my personality just as well and be a tad more stimulating.
U.S. horse racing.
Guinea pigs, although I don’t think I’ve ever answered an actual question on that.
So, the new movie G-Force - they don’t let the celebrity guinea pigs do the stunts, right? They use stunt guinea pigs? Fight our ignorance!!
:D:D
(yes, I know they are CGI…)
I’m NinetyWt’s backup. Gotta have redundancy in the system!
I don’t have the background in flood insurance or dams that she does, but I did stormwater engineering for ten years, and I’m currently working towards a PhD in hydrology.
: turns green with envy :
Which school? 
I’m supposed to be an expert in Philosophy.
Fat lotta good it does me here or anywhere else.
The law of U.S. federal income taxation.
Wow, there is a wide range of expertness here. I guess that is one reason I’ve stuck around for the past 8+ years. Always something new to learn in GQ.
Health insurance, life insurance, and annuities - not that it comes up much. Also contact juggling and in-universe Star Wars stuff (the latter hardly means I stand out from the crowd, though).
Not too long ago, I would have said I considered myself very knowledgeable about correctional psychology and sociology, although my attention on those fields has waned significantly since I’ve left college.
Nothing. I’m an expert in nothing. Well, I’m becoming an expert on the U.S. vet school admissions process.
I like to flap my jaws (flap my fingers on the keys, I guess) about horses and equine medicine, but I really know nothing in the grand scheme of things. If I’m lucky, I’ll be a minimally qualified expert in the late 2010’s.
I am cultivating an interest in the U.S. healthcare system and health care policy, but I’ve got a long way to go. Maybe in 2016 or so I’ll be an expert.
Oh, I have a basic degree in puns and wisecrackery.
In other words, I’m just a commensal, not a mutualist.
Film Photography, Desert Survival, River running and, of course, satisfying a woman.
Biologically plausible neural networks
Cognitive modeling (“computational cognitive neuroscience”)
Transhumanism
Linux
Wikis
I’m reasonably well-qualified to answer questions relating to the hotel industry, the restaurant industry, bartending, sales/marketing, and starting/operating businesses. Seems pretty lame, compared to all of the -ologists and -osophists around here! Like others have mentioned, though, I also have wide-ranging and oddly random interests which may someday be useful for something other than bar trivia…
Education of gifted kids, especially math and science but also across the disciplines
Non-fiction writing: education, popular science, skepticism and natural history - especially spiders
Primary orality - the way formal knowledge is encoded in oral cultures, especially natural history (my PhD topic)
LOL, although I do like the term “celebrity guinea pigs”.