I actually don't have a checklist as such. At this point, I animate kind of in a flow, and I stop when I think it is all feeling good and complete (or when the director/supervisor thinks it's all good and complete). I think at some point the principles of animation and some other acting stuff becomes a second skin. You work with them not exactly “thinking” of them all the time. It's like when I learned how to play the viola (a musical instrument that looks just like a violin, just a bit bigger with a more velvety sound). You learn a million technical things for a long time, but at some point it all becomes natural and you just play without thinking too much. Animation works in a similar way.
That being said, of course there are things I pay special attention to, and I think they have to do with the difference phases of the animation process. When I am working on blocking, my mental checklist is something like: Is the idea clear? Is the character consistent and are his motivations and thought processes coming across clearly? Are the poses solid?
For the poses, I usually check the line of action, silhouette, twinning, balance, asymmetry, and contrast. When blocking, I also pay attention to the rhythm and phrasing of the scene, just like I would with a piece of music.
Then let's say that my blocking was approved, and the scene is going to be taken from blocking to first animation. In this phase, I usually work on my holds and my transitions. Timing, spacing and arcs are areas that I pay special attention to. I make sure the weight shifts read right. Then I start to break things down, adding overlapping and more detail here and there, until everything flows nicely. If it is a character with hands and a face, I block them and on the first pass animation I add more detail – finessing the hand poses, maybe adding a blink here and there, maybe sculpting the important face moments more. This will of course vary a bit – if the shot is a close up, for example, there will be a lot more face work from the get-go on blocking. Anyway, I think you got the idea so far. I would keep working on my transitions and holds, spacing and arcs, weight and fluency, and getting the acting as nailed as possible.
For the polishing phase, if things are going well, that is the time to make all the details come together. I carefully check the lip sync, and work any areas that need a bit more punch, contrast or clarity. Face animation is refined during this time. I refine the hands until they are completely done. I make sure the overlap and flexibility of the character is working properly – things like dangling ears, tails, clothing – it all has to work perfectly. I also check for any intersections and basically “clean the scene” before sending it to the lighters.
The process varies a bit from scene to scene, and depends on the evolution of the shot -- the feedback I am receiving, how the scene is fitting in the cut with the scene that comes before and after, the technical challenges of each particular scene, etc. But I guess what I described above is more or less what goes on in my mind as I work on a shot.