race and religion are no no's with me.
-dark mutters- Buildings. But I've been able to work around it sometimes but uh, historically and detailed buildings like those in major cities have been a huge problem for me because we don't...really...have major cities here (aside from Milla de Oro and they're all modern) I've been to large cities like NYC, Vancouver, and Portland so that's where I've gotten my references but...it's not the same when it's not something you're exposed to as often. (Arquitectural Art Deco, I will master you one day...)
I'm thankful that Cosmic Fish is set in a Puerto Rico-looking environment from the early century because it's mostly forests and small towns minus exceptions (which is a fort city ha haaaa) but I still want to escape my comfort zone. I didn't know how to draw trees before Cosmic Fish and am slightly improving, so who knows, you know?
Anti-religious or yaoi themes is what I'll never do.
While I can't do mecha for now, I love robots and I desire to make a series with a lot of mechas one day.
Lots of small details, had to cut down on them because they don't mix well with arthritis and were destroying my joints.
Also, colouring is a very painful step for whatever reason, so I can't do a lot of colour work anymore. I still like to throw a picture in here and there, but I can't do a fall colour comic very quickly anymore.
Things I can't draw:
Certain poses (even with references there are some poses I struggle with...)
certain machines I find difficult (guns, cars, etc.)
Things I won't draw:
anything NSFW
beach scenes
basically anything considered as "fanservice" I refuse to draw.
Most stuff with a reference I can draw in some capacity, might take a long time, or need to be simplified. Backgrounds I still struggle with just because I'm not used to it yet. Cars I definitely could not do free hand in any way.
If there was a good reason I don't think there is much I wouldn't draw aside from some real messed up shit.
Could not: super complicated machinery, Iron Man, things requiring good symmetry, etc.
Would not: explicit pornography, extreme gore/violence
Sam I am: I am Sam
I won't draw anything really graphic sexually, I'm not big on drawing nudity in general. I tend to not draw a whole lot of little knick knacky things in environments too because of time constraints, although I wish I could since it'd add alot of depth to the characters. Maybe as I get faster I can work up to that. I also avoid drawing crowds, lol! And exterior city shots are a nightmare XD Exterior shots in general are not my forte! Zenchav1 starts out with mostly exterior city shots and I didn't really get a good grasp on doing BGs for it until the end of chapter 5 so it's kinda painful to look at now, lol XD
I can draw today everything, not perfect at all, and need references for a lot of things. But I can.
Other hands I will never draw homossexual sex, personal conscience. Some types of extremely violence. And I think is it.
There's really nothing I wouldn't draw? As long as I've got a good reference, I'm fine. Imo, it seems like a bad idea to limit yourself as an artist because something's too difficult to draw (if it makes you uncomfortable to draw, thats a different story!) Really, there's only like one or two nsfw-related things I wouldn't draw, particularly because they squick me.
Things I DON'T like to draw...front views of faces, I have such bad symmetry.
I'm the type of person who usually 'tries' to go beyond their limit (and fail miserably) so there really isn't much themes I wouldn't draw. I wouldn't really want to draw something that involves sexual violence and extreme violence. Tbh, I'm fine with drawing gore and sex separately but definitely not together. Errrk.
But in terms of actual skill sets, I am definitely limited by my inability to draw complicated machinery and old intricate architecture. While I do want to draw such things, I often find myself unable to do so because tbh, I don't really practice it that much.
There's a bunch of stuff I can't draw. Motorcycles, for example. I'm also not the greatest at drawing huge robots, or people kissing, or that impossible camera angle of a person's face from below.
But give me enough time to find a good photo-reference for it, and I can make something that doesn't look half-bad, and learn some things in the process.
As for what I won't draw, the list is mostly - explicit pornography, extreme gore/extreme violence*, most types of bodily waste (no illustrated poop jokes here XD).
*) My comic contains both violence and blood, but I have no interest in drawing super-detailed epictions of wounds or internal organs. It's very, very unsettling to me, and I usually don't read comics that contain gore either.
I don't like to limit myself on the grounds of what I think I can't draw. If I don't know how to draw it yet, then I'll just learn and practice until I can.
As for what I won't draw, sexually explicit content is probably the main one. I can and have drawn it, but I just don't enjoy it. I'm not a big fan of very detailed gore either. Still, there might be circumstances where I would draw them anyway, depending on the circumstances and the exact nature of the content.
I wouldn't say there's anything I just wouldn't draw, most of the time, there are just things I don't draw because I don't need them for my current comic. Otherwise, I'm down for learning to draw anything!
I could draw most stuffs as long as I have some good reference for it. Except if that said reference is not of the same camera angle that I want to draw then I'm pretty much screwed...
Stuffs I won't draw: excessive porn/sex scenes (especially when sensitive parts are showing), excessive blood, gore, any politically or religiously influenced subjects that might cause war and violence, anything that goes against my own principles and beliefs, and yaoi...
Backgrounds, mostly backgrounds
It really difficult for to decide on how detailed the setting is going to be because I really can't help to put in a lot of work in to a signal building and there's a lot more that I have to do. I also love architecture so when I do work on buildings I sike myself out thinking that it doesn't look that good so I work really slow on it.
and then there's hands... but thats because we see them all the time and we perceive them to look better than they are while drawing
I will draw almost anything, but there ARE some things I don't think I'll ever draw. Although, you never know, you can't read the future.
The first thing I won't draw is hentai or porno. I just don't feel comfortable drawing the naked parts, but I won't judge those who do. I just... can't.
The second thing I can't draw is realistic art. I'd rather draw toons or manga (although i'm still trying to draw manga)
I don't think there's anything I couldn't be able to draw, if I'd put my mind and patience to it. However I have no interest on drawing porn and would probably fail terribly at making it look passionate/interesting 
about the only things I wouldn't draw are yaoi, gorn, super duper hyper realism, huge mecha things, or anything "trendy" that doesn't interest me i.e Five Nights at Freddy's.
Not too sure I would draw a hentai manga. If I needed the money bad however, then okay hentai it is.
I don't draw the porns anymore (used to get asked for it a lot via commissions) it makes the significant other uncomfortable and I was never into it anyway (was probably drawing every thing wrong too since I actually have any interest to go looking it up either), I just needed the money.
Also not allowed to draw twincest (even if it's clean), because they are a twin and it makes them really uncomfortable.
Not that they dictate what I can and cannot draw, I just don't like making them uncomfortable.