L4D was my favourite game from 2008. It just felt like it was a product of a lot of focus and attention to detail on the small intangible things that mattered. Coop's been around for a while but has rarely felt so engaging and satisfying imo.
At first I didn't have such high hopes for it, despite looking forward to its release and the hype. Zombie games weren't new and neither was coop, so why would a zombie coop game be exciting. But theres something about the player-player-environment dynanmics that works really well.
It actively encourages complete strangers to think about teamwork and strategy and keeps things tense/challenging - which sort of reminds us of how we might feel if we really were part of a zombie apocalypse with 3 strangers. Forced to think and act differently with other people due to the unique situation.
It shows up different characters within the players like the guy who likes to stand back behind the others but forgets to keep a check on what might jump out from behind him - or the guy who feels most comfortable being the clear leader - or you might get the selfish/impulsive/reckless type who prefers to go lone wolf etc etc.
So depending on who you play with each game feels slightly different.
I thought it had tonnes of replayability - feeling like it'll be one of those games that have the same maps played over and over for years eg de_dust in Counter-Strike. And of course there will be mods.
It still needs better matchmaking and comms system though on PC.
I say Fallout was overrated - but I might change my mind later on.