I am a huge fan of L4D, and L4D2 was even better in my oppinion. i think the thing that makes L4D2 dissappointing for players is two-fold. Firstly that it doesnt follow the rules of most sequels. Secondly that the first game was designed in was way that did not lend itself to the making of a traditional sequel.
To the first point. The main thing that grabbed my attention, and was the main source of dissapointment for me personally was the characters. I didnt see a reason why the original characters needed to be dropped, and i still havent really gotten around to liking any of the new characters particularly. while that bothered me only slightly, i can see it being a major problem when u consider that games are, for the most part, character driven. You become Francis, or Bill, or Soap Mc Tavish, or Dominic Santiago, or Master Chief, or Gordon Freeman, etc etc etc, and through the course of the first game you become attached to them. Part of the reason a sequel is expected by people is that once u generat a rapport with a character, u dont want it to end. U want the next chapter, u want the 'further adventures of'. L4D2 pretty much immediately pised on that from the word go.
Secondly, i find that, yes, the structure of the campaign mode is pretty samey, but then, its just another 5 missions really, same as the first one had four. you can jump in wherever you want, there are none of the now almost obligatory between (or mid) mission cutscenes we've all come to expect. its a different kind of game, almost arcade like, i think. while something different can be fun, if its well executed (as L4D1 was). But a second helping of pretty much the same thing, only with chopped nuts this time, can disappoint.
The rational part of my brain tells me i should be disappointed with L4D2, and perhaps on some levels i am, but franky its too much god damn fun. its sad its not the 'further adventures' of bill, francis, louis and zoe, but its still damn good. and its got spas 12s! haha