Several indies are blogging about game length today so here's my two pence...
As a Player
I wanted to mention my opinion on this as a player because I think I am probably a rare case. I have completed about 90% of the games I own, and hope to complete the rest. Some of the games I have completed I didn't even enjoy. This is probably partly some sort of completionist OCD and partly the curiosity of a games developer who wants to figure out exactly what makes games bad and why.
As a player is length important? Yes: I prefer to buy games that are short, simply because if it's bad I'll want it to be over. When I'm unsure whether I'll like a game, I'll tend to opt for something that is shorter and costs more, than something which is longer and cheaper.
Although I'm an anomaly I think all gamers want to complete more games than they do. One answer is to allow a flexible length, and luckily side quests and DLC are a move in the right direction. They help me and everyone else to choose how much time it takes to reach the end.
As a Developer
I'm not content to make an empty experience or to dilute a game for length. Substream is going to be short but lovingly crafted. A quality-over-quantity approach, if you like. I know this isn't for everyone, and that's fine. Whether a game is "worth it" is different for different people. There is no one true length or one true price, the decision about whether a game is worth someone's money should be their choice, and based on what they want to get out of it.
Some bloggers are commenting on the tendancy for journalists to list a short length as a negative. I think journalists are acutely aware of gamers' desires for value for money (which is a good thing) but perhaps don't play games in the same way we do.
In particular, they probably don't get to replay games. My games are a carefully cultivated collection which I can easily view on my shelf and whenever I have the time and inclination I can replay any game I want. A journalist's games collection is a an ever expanding mass with less personal input, and new games must be played. I've bought short games that I've really enjoyed including Mirror's Edge, VVVVVV, Windosill; and I've completed these several times. In a high quality title there'll be things to appreciate that you might have missed before. If the majority of gamers really care about cost-per-hour of gameplay, then I kinda don't mind shortness being listed as a negative, but where that's done I'd personally like to see replay value discussed in more depth.