Of course they're taking too long. Just think of how they don't get episodic gaming right. but then again, it's not a bug, it's a feature, as they say.
They do a lot of play testing, and with that I really mean A LOT - I suppose hardly anyone is that serious about that. If a Valve game comes out, it's gameplay is a shining example of perfectly crafted, perfectly designed game - simply because they do this insanely high amount of playtesting and LISTEN to what the playtesters say. Play "Portal" with the dev comments on, and you'll see what I mean.
But I guess that's only a tiny piece of the puzzle. More important is, I guess, their structure. Or rather the relative lack of it. At Valve, there doesn't seem to be such a thing as a position or a specific job anyone has - everybody there seems to be involved in a lot. That's really good for the games they're making, because the final product has had A LOT of input - but of course, it's anything but a time-efficient method of making a game.
These two things are, I guess, what makes Valve so slow at developing games - and it's what makes their games so great. Can't have one without the other, it seems.