Start a thread. I’d like to see the logic on both sides.
I define expertise as a relative thing. What does everyone else come to you for knowledge about? Or if they were looking for info about something would you be the best resource immediately available?
By that definition, I’m an expert in the things I said. We could also throw spelling in there, but I would rather not. In other settings, no, I wouldn’t be an expert (especially kimono), but on this board I’m the one people ask (and I then usually double check my answers).
professional:
–Section 7 consultation related to the Endangered Species Act
–writing/developing NEPA documents in a lead role (as relative to cooperating agencies)
Normally I wouldn’t consider myself an “expert” on the above; however, given my interactions with resource staffers and management from other agencies, uh, well, I’m an expert.
hobby
–the works of Stephen King and collecting first editions (trade and signed limited)
–while I am in no way an expert on Korean cinema, I’ve probably watched more Korean movies than just about any U.S. white guy, and can discuss them at length with anyone, even experts from Korea (as long as the conversation is in English LOL).
I would have though scuba would have fit in there somewhere. 
Being mentally ill. I’m serious. I am fucking awesome at being depressed and anxious. I experience it daily, know it inside out, call it to the floor, deconstruct it and nail it to the wall before it can even think about doing real damage to my life. I can be in the middle of a crushing major depressive episode, be in actual physical pain from the intensity of the feeling, and still rate my overall quality of life as an 8/10 on the satisfaction scale, and mean it. I am the Master.
For related reasons, I am also pretty damn knowledgeable about psychology in general. A lot of it came from self-help research and a lot of it came from the fact that it is my husband’s greatest passion and a genuine academic interest of my own. We talk about it all the time. I still don’t know as much as he does, but I’ve picked up a lot.
Also, Nietzsche. I’m no Walter Kaufmann, but the fact that I know who Walter Kaufmann is and why he matters has to say something.
For work:
IC design, more logic than circuit. I’m reasonably famous in one subspecialty of this field.
I’ve become way too familiar with running conferences, particularly as program chair. I can create what looks like a coherent program out of a collection of random papers in just minutes.
Hobby: 50s and 60s (and the decades before and after) science fiction. I have almost complete sets of all the major magazines from 1945 onwards, and a fairly good percentage of the books published in the '50s and '60s. Don’t ask me about current stuff - I’m so far behind I hardly get anything current. I also have a fairly large collection of old books on sf from this time.
Me?
Most things automotive.
Improv
Indie Music
Bob Ducca
Track and field. I just took third (out of 3000+ people) in an online pick 'em contest, somewhat similar to a fantasy league. And a local prof researching women’s college track in the pre-NCAA called me before anyone else.
Some of you I would LOVE to talk with…other would put me to sleep…but a pretty freaking diverse group all in all.
Me…I am a lay expert at Long Distance sled dog racing; Think Iditarod, Yukon Quest and other…especially proficient in race history and current mushers.
Also pretty good at Grateful Dead and related band history/trivia/scene knowledge…not a Rhythmdvl but pretty close. 
Definitely and expert as a General Manager for my company and Bakery/Cafe…
tsfr
And being awesome.
Expert:
SQL Server specifically replication
Hobby:
Electronic music and synthesizers
::: Blush:::
I hesitate to call myself an expert on anything, but these come closest:
Photography
The Beatles
Adobe Photoshop
Adobe Lightroom
60’s music
Home repair
Medical stuff
mmm
Get a room! 
Data compression algorithms; construction of certain types of puzzles. Various computer engineering areas (but of puzzle-solving rather than knowledge-based type).
I really am smarter than I seem. 
I have more U.S. patents than I have teeth, and the best circuit designer I’ve ever met called me the best circuit designer he’d ever met.
Really? What with your username and all, I’d imagine there are six people better than you.
East and Central Asian history, religion & linguistics
Phonetics
Speech pathology/audiology
Lesbian porn
rock music
the writing of Jim Thompson
alternative comedy
classic foreign films
keeping track of multiple lies I’ve been telling without giving myself away
I am good at a lot of things though I can’t make the claim of being an expert at anything… too repetitive for my nature.
‘Get good, and move on’, as I say.