Experience tells me beta readers are crucial in this. What did my writing over/underdo? To be honest, I didn't know until I had other eyes on my drafts. It's not even just that I don't know what my personal weaknesses are; it's that every individual story has problems no writing tips can guess. Better still, you'll be amazed how little work it takes to fix the specific issues they raise, relative to how much good you do your work in the process.
Which characters aren't fleshed out enough, and in what way aren't they? Believe it or not, this isn't just about personality. I rarely describe a character's appearance, and if I do it's because they really need it, possibly to showcase an aspect of their personality; but maybe someone needed it and I didn't think they did.
Which scenes deserve to have their emotional implications better addressed? In one recent example, I used a first-person narrator, which I rarely do. This made it all the more vital that, when something that really mattered to her happens (for good or ill), I make that evident. In arguably the two most important examples, I didn't... until someone pointed it out, whence I did.
Which chapters don't belong? Yes, scenes or even whole chapters can warrant excision. It depends in part on how they got in there, of course. Going back to the aforementioned story, because the protagonist had originally been part of a more even-handed ensemble cast in a number of short stories, the realization that I should focus on her also meant deciding which stories to keep. When one pares down, one often doesn't initially pare down far enough.
Finally, what goes wrong in your sentences? Once you know what makes them hardest to follow, you'll come up with your own ideas for what to do as you redraft. Unfortunately, this one does involve a lot of work. But, for example, I was told there were too many sentences with difficult syntax (which isn't surprising, since the character was a prodigy). So I thought to re-read the longer sentences, and that also let me notice certain words padded sentences and could often be removed, even from sentences that weren't all that long or hard to begin with. You might be surprised how much you should gut besides plot detail.