One of the earliest game trailers I remember was for Planescape: Torment. It shipped with the 5 CD version of Baldur's Gate and when you first installed the game, you'd watch the trailer for Planescape which was due for release on the following year. This was in 1998...
http://www.youtube.com/watch?v=PevWsWNjSZ4
I remember watching this and thinking 'One day, games are going to look that good!".
At the time, developers did not have many options to distribute their trailers, so it made little financial sense for them to invest in one. Then in the following years, residential broadband became available and opened channels for delivering media on to an international audience.
Just six years later, the quality of trailers accellerated to rival those for Hollywood blockbusters...
http://www.youtube.com/watch?v=dYK_Gqyf48Y
Another six years on, there hasn't been as significant an improvement in the visual quality of trailers, but the rules for gaming have changed; Every epic game, needs an epic trailer.
http://www.youtube.com/watch?v=lc3Bzkgkbzg
This is the latest one I've watched, then I started thinking back to the original Deus Ex and on to the evolution of game trailers.