Perspective. Backgrounds. Clothes. 
Have you ever heard of the "draw a box" lessons?
It really helped me to understand perspective better.
The book perspective for comic book artists is good but I have a similiar problem,
I have 3 books about perspective but I never read them 
The problem is that perspective is probably "the" fundamental and once you
understood it everything else will be easier
It´s all equally hard or easy for me to draw the single parts of the human body, a lot of people say hands but
the principle to draw hands is as hard as other parts of the human body. I´m not good at it but also not horrible
and I enjoy drawing hands.
What´s really incredibly hard for me is putting it all together, attaching the arms to the ribcage, the legs
to the torso, the neck to the head to the torso. I want to control it and I have the feeling it is all floating
around at the same time I try to handle the proportions without making the figure looking too stiff.
It´s tough and when I see other artists it looks like they can move and bend their characters like they
want and it all fits together perfectly
I did "Draw a Box" twice. Traditional and Digital... * Don't hurt me mentioning "Draw a Box" again, please * 
I do believe that drawing in perspective implies a certain "mood" but this is me talking about things I'm still not used to which is thinking in perspective. It will come up more intuitively for me, eventually.
Twice? 
I failed to complete it twice but the 250 box challenge really helped me,
I´m trying it again at the moment but I´m stuck at the texture lesson
Anatomy and panel composition. I always feel that I could've make certain scenes flow better or have a stronger punch.
Easily mine is anatomy and posing. I would say it was environments, anatomy, and poses but I feel like I've gotten at least confident enough not to say that environments would be a flaw anymore. But I have been trying to branch out and try different poses and read books to improve my drawing.
I have a flavor of the week for what I'm unsatisfied with but if it's something that I think I can focus and practice then that's ok (right now I hate the way I drew arms, but I will fix it with some studying). In a deeper way I think my time spent to result ratio sucks for a wannabe webtoon artist. I'm going down a path with an inefficient art style and need to figure it out somehow.
Everything lol. Although everything is slowly getting better. I don't do will with faces that aren't straight on... I also struggle with straight on faces, but not as much. My body proportions are not consistent. I'm also not consistent when drawing the same character. I'm not super good at hands. Hair can be difficult. How do arms work? There's really a lot that I consider weak in my own art. However, I do enjoy my art, and I do like most of it. It's just not where I want it to be yet.
Oh, good question. I think I have issues with posing, particularly posing with foreshortening. And the bottoms of shoes/feet. Its a struggle. XD additionally I think that my 'clutter' placement in environments could use some work. like when i draw an office, 'WHAT DO I PUT IN HERE?' so, trying to figure out how to make a messy environment, rather than an unnaturally clean layout.
Ahhh yea, there's always that something that doesn't quite work, huh? =w=
Right now I'd say.... For awhile, I'd overdo it in a way? And it'd detract from what was the important part of the frame. I'd say that's where I'm working at the most right now- making things pop but still work together.
Backgrounds, by far xD
Later, in a close second will be coloring. Or maybe just shadowing. The colors department right now is fine, but adding shadows its still a problem with me. I still don't find a technique that I can feel confortable with, so I just stick with the common cell shading.
Lastly, only because I have been practicing like crazy with the comic, it's anatomy and perspective. I can't have enough practice, though
For me, I wish my lineart was cleaner. I'm pretty good at lineart overall, don't get me wrong, but there's places on there where I'm like eesh, so messy (but I post it anyway because I gotta finish this thing)
Foreshortening and proportions. Whenever anything's foreshortened, something just looks off. I do it often to practice but I can never get it just right. And the proportions of my chatacters are never consistent, especially on my more realistically drawn characters. Heads are either too big or limbs and torsos aren't long enough. It's so frustrating.
I'm not very good at blending shadows. Somehow a lot of blending or soft brushes tends to make everything look somehow fuzzy. So I started using a more crosshatching style for my shadows and even highlights. I think it has a neat look. But definitely covering a weakness.
For me, I could probably stand to improve drawing things like horses, or better differentiating faces between characters (aka 'same face syndrome'), and I always struggle with the right level of detail in my backgrounds (how much do I need for it to not look like a cheap afterthought, how little can I get away with), but the thing I'm most frustrated by...
... is things I like about a drawing getting 'lost in translation'. You know, when your sketch looks great, but somewhere between cleaning up your linework, inking, coloring, and adding finishing touches... something just... gets lost. No idea how to fix that, and it's so, so frustrating.
