The cool thing about thinking of animation as a language is it helps me break down what I'm trying to do. First off, an admission. I love metaphors and allegories. Just love 'em. They help me understand things. I think they're great teaching tools. Anyhow, the allegory I have in mind here is that animation is like writing. You have handwriting, spelling, sentence structure, paragraph and story structure. Each level is a slightly more global look at things, but it always comes back to my earlier point: having something worth saying in the first place.
I think of my clean up and polish stages as basically akin to the effort to make your handwriting clean, smooth, easy on the eyes- pretty. Great handwriting can't fix poor spelling or bad sentence structure, but it looks nice. Just like great polish on a scene can't rescue poor acting choices or weak poses, etc. Clean up is only as good as the foundation upon which it's built. Which is why I think of it as handwriting. But bad handwriting can make a great idea illegible, so you can't scrimp on it, either. Still, it ought not be the focus of what you're doing, merely the product of a professionalism that drives you toward excellence. Polish/handwriting can't rescue bad ideas. It just makes bad ideas pretty to look at.
Spelling is similar. You can comunicate an idea with misspelled words. Its just distracting. So weak poses, funky arcs, odd motions or less than great timing show the same kind of lack of professionalism that misspelled words do. People still get the idea, but it just feels amateurish, rushed or sloppy. It certainly helps to spell things correctly. But again, spelling doesn't rescue bad ideas. It just makes bad ideas easier to read.
When it comes to unique and sincere acting choices, masterful timing and great poses I think of these elements like sentence structure. It is foundational to what you're saying. The stuff has to flow. Your acting choices, your pacing- it all needs to work together, one idea flowing naturally and seamlessly into the next. It has to be easy to read, natural, concise, clear. (unlike this post!) Avoid run-on sentences (timing with no texture), wrong words ("don't jump to contusions"- a pose that isn't appropriate for the moment), poor structure (acting choices that don't flow), etc. Here we're starting to get to the point. This is where you need to focus your energy because this is the core stuff of what you're saying. Choose your words carefully. A poor choice of words can cause wars. By skimping here you can utterly ruin what could be a golden moment. Choose the wrong words and state them in the wrong way and it'd be like proposing to your girlfriend as if you were Andrew Dice Clay. You dropped the ball, buddy.