1) Brotherhood and Revelations both didn't just append the story with the next chapter, but added new gameplay mechanics and even an entire multiplayer section, rebalanced the game, updated the graphics, and changed the scene to an entirely new area. that's more than most expansion packs do. heck, there are sequels that have done less.
2) both the brotherhood and revelations folders are larger than the AC2 folder (8 GB compared to 5). and I don't know about you, but I have second thoughts about 8 GB DLC.
3) brotherhood and revelations both were much longer than what DLC normally adds. for example, the missing link (Deux Ex DLC) adds about 2-3 hours, depending on how you play it. Brotherhood for me added something like 30 hours, and revelations was half that for reasons below.
as for my personal opinion, I really hope that the combat from AC:R won't make a return. If i stab someone in the gut 5 times with each of my hidden blades or double slice their throat, i expect them to die, not to still hang around with 3/4 of their lifebars still remaining. it just needlessly slowed me down, was unrealistic and cost me my life more than once. and i'm glad den defense won't come back. overall, i found that AC:R was just not my idea of fun. the combat felt way too slow, i didn't find the missions that fun or exciting (collecting books? really?), and was it me, or were there no sidequests for other factions?