Google Caffeine is a New Search Algorithm factors are given below: 1. Speed 2. Accuracy 3. Temporal Relevancy 4. Index Size
I recently noticed a major drop in indexed pages as the result of caffeine; mostly on pages with little content were affected (e.g. pages with a fews lines of text and about 20 links). What I did was simply add more content and they were back in the index within a few days. Also the pages that were linked from this page were back in the index. The amount of newly added text was just a few lines, but seemingly enough for the threshhold.
Have to say I'm very impressed with Google Caffeine so far. I rely quite heavily on social bookmarking and blogging and finding things like blog posts and Tweets appearing in the SERPS almost immediately is very encouraging.