Well as I see it, it's what they do with it that counts, and if pushed I would say that it's a stupidly big expansion pack rather than simple DLC. most expansions can't even dream of making so much content. I mean, compare it to Crysis Warhead which runs better, looks better, has better AI, has a better campaign and is almost as long as Crysis vanilla. I would say that's an amazing expansion pack. And yet, L4D2 does all this plus adding (Fantastic) weather, and tons of minigames (inside the Dark Carnival campaign) and being longer, harder and more polished in almost every way, and each campaign having something special and impressive in pretty much every map of every campaign.
The amount of content is what made them decide to make it a sequal, not the tech but the weight and impressiveness.
Oh yeah, and there's not much point going on about how they're using the same engine with improved script, since they've been using that engine since Counterstrike: Source. It's a modular engine, they're gonna use it for as long as possible.