There were a couple of things about the game that made it a lot of fun to play:
The gameplay offered up more realism than most games of the same era, yet it wasn't overly realistic. It stuck a good balance between arcadey fragfest and super-anal-realistic simulation. The whole one-life per round thing was kind of new at the time and made it a more difficult game to master, as well it became much more necessary to support (and be supported by) your teammates.
2) Continued development.
Every once in a while, a new version would come out, with one or two new weapons, some new maps, as well as some upgrades to existing favorites. The development team was always working to improve the experience, which is not something you really see in most games, especially FREE ones. The addition of an excellent in-game real-time voice-communication was a feat that has yet to be duplicated, and it added a whole other dimension to the game.
I kind of miss playing CS, but in the same way that I miss Q1; a kind of nostalgic reminiscence of the good times had while playing. I have some hope that any future sequels will be as much fun as the original, but a part of me suspects that what made the game so great at the time was that it was fresh and new, and that original feeling can't ever really be duplicated.