Like a film, I usually find it's about pacing. I could probably write a lot about it, but I'll save you the torture...
Generally, I'm drawn to games with some sort of narrative, so there's a real need for a decent balance there. There has to be enough to keep it interesting. For example, I don't think Arkham Asylum or Arkham City were particularly long games, but there's a well-planned feed of new ideas and locations or other changes so it never becomes dull. That's even without the side stuff (that I also loved).
My gaming habits have definitely changed though, as I find myself with less time (or inclination), so certain games appeal to me more. I can quite happily leap into Mass Effect 3's multiplayer for an hour or so, or do a few side missions in Borderlands 2. Just recently, I've been alternating between doing a mission of Halo 4 per sitting and some Assassin's III side missions or, again, the occasional longer, more involved mission. I wouldn't dream of starting a game like Fallout 3 or Skyrim, or many RPGs, really.
I can't remember where it was said (some gaming site), but someone said Arkham City felt like it was designed by people with children. As in, they understood how to keep the moment-to-moment, bite-size gameplay rewarding as well. It's a valid point, I think, and I can see where they're coming from. I remember there was the Bungie '30 seconds of fun' concept, but I never found that an accurate term (and that's without getting pedantic over exact timing). There's also the 'review factor', whereby reviewers gaming habits may be markedly different from a player. They will have sat down and played a game all day, finding it too repetitive or too short or whatever, whereas someone taking their time won't be bothered so much by certain things (AC3 would probably fit into this).
Anyway, I think it would be useful if people expanded on why they felt a game was too short or too long. In the case of the latter, it's easy to say that things dragged on, gameplay or storywise, but when it's short it's always related to cost which, as FDS already mentioned, will usually change.