As usual, Ed Anon says exactly what I think, only better.
Here she talks about what can be fixed easily, what can't, and why some authors who "fix" things drive us to drink.
I should add to this by saying I am not an editor, nor an "editorial minded agent." I've never been an editor, and after seeing the work of some truly talented editors, I DO think they are superheroes.
Which means if something needs to be fixed, even if I know what it is, which I often don't, I'm more likely to say "not for me" than "here, let me give some suggestions." Not all agents are like that of course.
That is one of the top five reasons you should query widely, not offer exclusives, and not obsess about rejections. Agents bring different skills to the game. Not all agents, even spiffy sharky ones, are the best agent for you.