I think character development is quite important, especially for the protagonist? I wouldn't prefer a protagonist that never learns her/his mistakes throughout the story and never matures. Besides, development doesn't have to mean changes in traits. It can be a change of perspective or approach to overcome the shortcomings.
I mean character development is a good thing, but some story work better focus on its strenght. Most xianxias is all about fight, cultivation and adventurer. character are very unidimensional, in special the side character, the loyal brothers are always loyas popcorn cheering the mc, the jade beauty after being conquered is forgotten in a corner, even if the mc go out of his way to improve their cultivation they almost never do anything useful. There is no way to expect much development from that. However if the core of the novel that is fight-adventure-cultivation start to become stale is the moment people start drop, the same goes if there is a great change on the mc personality. That goes back to the "Promise and tone" problem. However, if you can keep the core points good then having character development will make your novel rise from mediocrity to something good.
Don't procrastinate. Try best not to switch back and forth between past and present tense. Keep it consistent. Don't jam everything into one long sentence when it could be split into a few sentences. Show and not tell. Makes it smoother to read when its more show. Telling makes it kinda choppy sometimes. Don't have too high expectations. Everything starts from somewhere at first.
A really bad habit is to overthink, just go with the flow of how you imagine the scenario to go then make the fixes after u finish or if u aren't satisfied with how it went just keep writing and writing and don't stop esp if u hit that wave of inspo you can always get back to ur plot holes and mistakes after.
I would say the habit to avoid is mindlessly using cliches or tropes. I'm not telling you to never use cliches or tropes, in fact in some instances or type of stories it might be crucial to use it, but what I mean by this is to always think deeply about the cliches or tropes, their purpose narrative-wise, and why it works, after you deconstruct the cliches and tropes where you understand its intricacies and why it is used that way and the effect it is to the story. Then, you can use cliches and tropes organically that FITS your story, instead of just another copy-cat story.
The worst is... To not write Write, write and write. Writing is like a muscle. The same goes for drawing. Careful about the tense. Don't think your readers are stupid: show, don't tell (okay, maybe some are idiots but...well...) Grammar. Punctuation. Hmm... This is really personal, but I don't like contractions. If it's in dialogues, fine, that's not a problem. But this might be because I'm speaking languages that don't allow this...