Yes, I think a proper trail would take a while to make. As Olaus mentioned, snippets + dramatic music = thrilling excitement. Abet, that also means you'd need many games with lots of short video clips from many perspectives at different points of the game.
Concerning an intro video in game, I admit, I'd love to see one (whenever I buy a new game, I usually always watch the intro video first just to see the first impressions. The graphics here are usually unparalleled (and usually better than ingame itself, but who cares? It attracts the gamers attention and they like it.) However, that is a more longterm thing, as distributing a high quality video file with Glest can double the filesize, and there's not much for free video codecs that will do well in a c++ program (obviously commercial programs can use the amazing Bink, but a free, open source program? Nooo). And overally, it would have to be very high quality. Abet, this is offtopic, so moving back to a trailer...
Since it would take a while to make a trailer, the current videos work fine, though I do think we should be more ambitious and aim for an innovative trailer that will really catch the eye of potential downloaders. It doesn't have to be too long by the way. Most trailers are a mere minute or two.
