I used to be a really big AC fan. I liked the first, absolutely loved the second. I found Brotherhood to be quite good actually, I felt like it had enough changes to it. Revelations was okay, but the story was the only thing keeping me going there, I was starting to get really bored with the gameplay. AC3 had changes, but it ended up turning out relatively the same as the ones before, though I had more fun with it than Revelations. I'm not interested at all in Black Flag though, not one bit, even though I really liked the naval warfare in 3. I don't know, it's annoying. I don't really have a problem with sequels not changing much, that's not a big deal, it's when they pump the sequels out yearly without changing much that annoys me. Ever since AC2, we've been getting an Assassin's Creed game yearly, and to be honest, it's getting really boring. Maybe it comes with the setting change too, Constantinople was shit, easily the worst setting in the series I think. Colonial America was interesting for a time, but nothing really captured me like Renaissance Italy, or heck, even the Crusades. The setting matters quite a bit, alongside the fact that, while I like Ezio, and think he's the best character in the series, he didn't need three games. AC2 and Brotherhood would've been fine, I actually found the Altair missions in Revelations to be the best part of the entire game.
I guess the tl;dr version is, COD gets more flack for being bigger than AC, but AC is just as bad about it as COD is. And, if you ask me, it won't be long until AC starts getting the same amount of flack that COD is. Ubisoft seems okay with running the fastest growing game series into the ground.