Honestly not much has changed in the end, but its still an insane step up in every possible direction from the first game.
OK, NOT trying to be an ass, but isn't that a contradiction? Personally I haven't seen what I would call major new developments. L4D2 does offer something new, just not $50 new IMO. If the game had launched @ $30, I probably wouldn't have bothered posting to this thread. I guess I just see a ton of untapped potential, and add the fact that Valve committed to content updates to L4D1 that ended up being paid content in a full sequel...it just leaves me a little let down.
It could very much seem that way, but I see it as more of they changed a bunch of little things, when all of those little things changed together it became huge. It's hard to explain but individually going oh this changed and that changed it's quite clear that not much has really changed.
How about adding non-hostile NPCs? Where are police/army forces that can occasionally assist you? How about a whole campaign designed around rescuing helpless uninfected? Where is the 'Find the Cure' campaign? I realize certain campaign designs would be entirely unbalanced for Versus mode, but why should that hinder a talented dev team unless they just have no interest in evolving the game? Personally I'd rather have some unique campaign design rather than the exact same gameplay in every 5-map sequence.
None of that yet, really we are very early in the story of what has happened just 3 weeks since the first infected. The game starts off with you running to the evac station and the helicopters taking off before they get overrun with zombies leaving you behind. You find out in the first level via the whiteboards etc that New Orleans is the only major city that is not overrun, leaving you to go seek it out.
The campaigns are much much much more unique in this than the first L4D. But ultimately yes its basically the same you just have to get to the end, no one else is alive near you.
I haven't completed the game yet so I don't quite know how it ends or anything but right now the military and CEDA are arguing, CEDA still thinks that they can contain the infection. The military wants to get rid of the infection the easy way.