Since I don't have that much time on my hands to spend on my artwork these days, though I've never particularly felt like I've had enough time, there aren't many things I would devote hours to. The two things would have to be 1) lighting effects and 2) tattoos (for one character specifically).
When I say lighting effects, I don't mean highlights on like objects and clothes, I mean more in line with special effects like with magic or water.
For example:
The fire and water effects I do when drawing my comic. I spent a lot of time trying to balance making them look believable, but so detailed that it would take too much time to draw.
Same thing with the fire and ice in this scene.
These two didn't take as long to do in terms of time, but there was still plenty of thinking involved in how they would work.
Next, tattoos and body markings. I have a few characters with tattoos, and unfortunately I'm a person obsessed with consistency. So if they have tattoos, no matter how detailed
or simple , I'm going to try to keep them as accurate as possible.