Was there a marked improvement from 2 to the other annually released "sequels"?
Actually I would say there was from 2 to Brotherhood which imo is easily the best of the lot. Revelations on the other hand, whilst still a good game, added nothing new except needless options which were only put in because they felt they had to make it different in some way!
There was simply no way to justify yearly games that have little difference, so they added micromanaging encounters and other pointless things like bomb recipes. Unfortunately, it is becoming very much like COD in that respect.
I would much prefer a new AC game every 2-3 years which significantly adds to previous titles. Yearly releases are not necessarily always bad, but this series is in dire need of a little depth, which the story demands but the quick turnover doesn't allow.