Has this been a thread before? I honestly don't know. Just read the title.
My opinion is that it's a fairly important factor, but shouldn't be taken into account too much, especially in milder cases. If I had a choice between an awesome game with a 7-hour campaign or a half-decent one clocking in around 60 hours both with no replay value, I'd probably go with the former. However, shorter and I probably wouldn't be satisfied with my purchase.
I also think game length should be relative to price when it first comes out. This is especially true for downloadable titles, as I see people complain about titles like Jett Rocket (4-5 hours for $10), Braid (5 hours for $5 (discount)) and Limbo (5-6 hours for $10) for being too short, and then comparing them to old PS2 titles that cost $3 now. Considering at the time of release, those games were $40, that argument is fairly irrelevant. And when most action games take 10-20 hours for $40-60 titles, I think people complaining over Limbo are being unfair.

JK some games are awesome for only a few hours