It's often said about revisions that you should "kill your darlings," i.e., get rid of the scenes that you love but that don't actually help your narrative move along. Love is blind, after all, and sometimes the lines, descriptions or dialogue you most adore don't belong.
But what about when you're in the midst of writing a first draft? If I've written a sentence that I really like, I check to see if it's out of place, and if so, in what way (tone, style). If it's truly better than the material surrounding it, that's a problem. But instead of freaking out because I like something, and viewing it with suspicion for being "a darling," I'm going to try to lift the rest of the prose up to its level. That's the challenge I've set myself while heading into the third act of my current project.
And when it comes time to edit the sucker, I'll see if I was on the right track, or completely insane.