Is coloring included in this? If so.. oh boy.
Here's my list of ones I also struggle with.
1) Hair. I do not have the skill or finesse to draw beautiful elegant locks of hair at this time so I understand this one completely. But yeah, I'd say it's pretty difficult to nail good looking hair.
2) Clothing. As long as it's covering their no-no squares, I guess it's a pass? Kidding. Sometimes people over-do the clothing though to a point where it makes no sense. Like how many belts does this character need to signify their badassery?
3) Horses. They're just weirdly complex sometimes..And people have drawn some freakishly looking horses.
4) Buildings (Exterior). Just. So. Tedious. I get it but sometimes it looks like people just gave up, especially on the perspective.
5) Dynamic poses. I feel like a lot of people struggle with this one, just making characters not look as stiff or drawing them from different perspectives, etc.
Other ones that I feel pretty comfortable with now:
6) Anatomy. This can be stylistic but some anatomy can get really goofy looking.
7) Hands. A lot of people try to hide these. Don't blame them. Hands are pretty intricate.
8) Feet. A lot of people ALSO try to hide these. But feet are gross so it's ok.
9) Expressions. Some characters just seem to have no emotion some times.
10) Eyes. The good ol' "I'll draw one perfect eye and cover the other one with hair " trick happens a lot.
I feel like most of these can be remedied by doing studies however, like just spending 30 mins- 1 hour a day drawing what you're struggling with. I did this with hands for months until I became happy with how mine looked. Gesture drawings have also been helping me with dynamic poses.