Here are a couple of progression reels from animators Jeff Gabor and Cesc Pujol.
What impresses me the most about Jeff's video is how much detail he puts into the blocking process and how much is trimmed before the final animation.
Ice Age:Continental Drift Progression Reel from jeff gabor on Vimeo.
Cesc's video is interesting because it shows a mix of live reference and thumbnails. Again, compare the difference in the subtleties between the blocking and the final animation and you'll see a lot of changes.
Wine Tasting Process from cesc pujol font on Vimeo.