Right mix of tmosphere is definitely necessary.
Having a unique concept helps a lot (but you can take an overused, beaten-to-death concept and make an extremely popular game out of it because, again, polish and atmosphere can play huge roles).
Length doesn't seem to make any difference about how much the game can sell. I mean, even with reviews and descriptions, you never really know how long a game is until you play it (Tales of Symphonia is extremely long, VVVVVV is extremely short).
Production value is a great way to hire those people who can polish the game (more detailed graphics, better-orchestrated music, programmers who can put in better effects, etc). But even people with no money can make a beautifully-polished game. Simple things like text fading in and out, menus swooping in from outside of the screen, voice acting, cutscenes, points attained hovering above character's head before disappearing (I think some Legend of Zelda games had the latest rupee attained hover above your head).
Hmm you make a good point about length not being a huge factor, past a certain threshold. Portal was only around 2 hours, being generous, but I thoroughly enjoyed it and did not feel ripped off at all (and it was a damn good deal with the orange box). The perceived uniqueness and originality of the game and the sheer quality of the experience made it worth it. Same with Braid, although Braid is much longer and denser than most puzzle games out there - with almost zero filler. So I think length matters, but in a very complex way beyond just "longer is better."
I guess when it comes down to it, presentation does matter a lot to the "value" of a game. People can kind of tell subconsciously how much effort has been put in a game. All those little details add up, and when they're not there, the perceived value is naturally diminished. With a game like "Trine," you can really see the love that is put into the art. Same with Braid and Portal. Little touches like the Companion Cube don't just happy accidentally.
There's also just the "coolness" factor. I'm all for SpaceChem, but it definitely does not have the "cool" factor.