The relevancy comes in with, how much do you want your sequels to stray off the path provided for them? Would you have preferred Invisible War to be more like the original Dues Ex? I would have.
I do agree that L4D2 probably could have went slightly more off that path, In which direction I am not sure but I wouldn't want it to wander too far.
Oh I agree with that, as a matter of fact I usually get called anti-innovation because I complain when sequels stray. I don't have a problem with L4D2 being more L4D and apart from losing a bit of mood and ambiance I would say L4D2 is a massive improvement.
I think a year just isn't enough time for me to need more Left 4 Dead. I was still playing the original somewhat regularly and the good mods were just coming out. It just felt too soon to me. Looking at it as an expansion makes that more "ok" but then I wish the games were connected instead of seperated.
In all I would even say I love the game, there is just something that keeps me from being as excited about it as the first.