Alec Meer at Rock Paper Shotgun laments the slow demise of decent game demos.
Demos still exist and probably always will, but they've become the exception rather than the rule. Even in the last couple of years, the decline has been rapid – it's a relative rarity to post about one on RPS now. Publishers seem to have settled on marketing and heavy, heavy promotion (often including bewildering ARGs) as the alternative – a surer way to drum up interest in and expectation for the game, and one that does so without the dread risk of a gamer discovering that, actually, they don't like this all that much. For some really big games, the norm even seems to be not releasing a demo until weeks or months after the full release, presumably to help drum up those few stragglers who somehow resisted the pervasive trailers and advertisements.
I still have some Amiga cover disks somewhere around here…
The Slow, Strange Death Of The Demo [Rock Paper Shotgun]