In no particular order: Spore, Little Big Planet, Sports Games
Spore and Little Big Planet started a revolution in my opinion. Those 2 games are cause for more and more games to allow more user generated content as well as being able to share that content at the push of a button (and an internet connection). On the negative side, but still defining of 2008, Spore's use of the DRM technology. We all have read the reports, reviews, etc. of the absolute outrage of gamers around the globe over DRM technology. As piracy goes, "DRM-like" software will continue to be developed, much to the displeasure of the honest gamer.
Lastly, I chose 2008 Sports Games because of the online features implemented. Two NBA games came out with "Living Rosters" and "Player DNA" that gave more of a SIM feel to the games and allowed people to constantly be playing an updated game. The fact that 2k Sports and EA Sports is able to accomplish this is definitely a feat that sports gamers should be proud of. Also, most of the sports games released in 2008 had online "team-up" features that allowed players from all over the globe to join on a team. The best implementation of this feature goes to EA Sports' NHL 09. That game allowed 5 people from anywhere to join a team and play another 5 people from anywhere in a fast-paced, lag free hockey game. Playing online when all the players are user controlled makes the expierence far more enjoyable as you cannot 100% predict what humans will do in every situation.
Bottom line, 2008 had many defining moments that I didn't mention; however, the 3 mentioned above are my personal opinion.