What I like is when the developer says nothing and just shows me 15-30 minutes of someone just playing their game. No scripted events, no cutscenes, no marketing hype speeches... Just show me what I'm going to be doing 90% of the time in the thing.
Every time I see an ad for a game that does not show me gameplay but has some voiceover explaining how cool it is, I just assume they didn't show it because they know it fuckin' sucks.