I rather play Splinter Cell: Conviction (takes a day to finish) than Assassin's Creed 1 (takes a week). The quality of experience is more important than length. If a game is longer due to more quality content, fine, but length doesn't equal good in it's own. If a game is short due to lack of content, then it's still the level of content that is bad.
The fact that quality is more important than length, doesn't mean length isn't important. Splinter Cell is an ok game, and I think that last one would be a lot better as a longer game.
Assassin's Creed was a one trick pony, and it was crap after 30 minutes - when I realised that it was just endlessly repeating itself. I don't care how short or long it is, I'd never play it through to the end, regardless.
Length is important to me, and I mean VERY important.
Can I enjoy a short game? Yeah, it's happened - but it's very rare, and I actually stay far away from short games, because I know they're not for me.
Again, it's a matter of taste.
Some people don't mind short games, and they don't need perspective and a sense of being able to enjoy a game for several hours.
Some genres can handle a short length better than others. Something like Dungeon Siege, specifically, could never appeal to me as a short game - no matter how brilliant the gameplay was.
Rather, it might appeal to me - but it would still be a LOT worse as an 8 hour game, than a 30 hour game. I'd rather have content "spread out" for 30 hours, so I can enjoy the gameplay for a longer time. But nothing is that simple, and it depends on your vision as a developer.
When I play "designer", the last thing on my mind would be a short game.
Anyone truly believing the original Dungeon Siege would have been better as an 8 hour game, with all the "content/story" placed in those 8 hours - are not thinking this through. The hack and slash genre doesn't lend itself well to short games.
That game suffered because it had incredibly simplistic gameplay, and a non-existant story. Chris Taylor is a moron as a designer, if his conclusion is that the game was too long.