Two things I wish that writers wouldn’t do:

Tell readers what, and how, they should think about their books.  Believe me, I understand the impulse.  One of the hardest things to accept, if you’re a writer, is that once your story is out there loose in the world, you have absolutely no control over how other people incorporate it into their own heads. The readers who excoriate you for crimes you had no idea you were committing are bad enough; the ones who really like your books for reasons you find repulsive are even worse; and sometimes the urge to tell everybody that They’re Doing It Wrong becomes well-nigh insurmountable.

Go back and rewrite their earlier works to make them better.   I can understand this impulse, as well.  We all like to think that we’ve improved in our art since we started working at it, and our novice-writer gaucheries can make us wince.  But rewriting one’s early stuff to bring it up to standard doesn’t usually improve it enough to make it worth the loss of the energy and reckless endeavor that often characterize newbie work.  (I know there are things that I tried to do, and at least came close to carrying off, in my early stuff that I wouldn’t attempt to do now because I know how low the odds are for success.)

As for writers who go back and revise their earlier work to bring it more into line with their later political or philosophical convictions . . . they depress me.  Sure, you don’t think that way now, I want to say to them; but an earlier version of you once did.  Trying to bring those thoughts and words around to the current standard always strikes me as like trying to kill that earlier you.