Please explain some of the generation-defining titles or moments in games that you have experienced.  Don't list, and don't worry you can always come back and add more later.

These are my first three.

My first one was Call of Duty 2 for Xbox360.  The immersion with the HD graphics, the surround sound, and the multiplayer opened up this generation for me to think that this was not "more of the same".  Instead it was "the same with more."  It already looks a little dated, but the series started off this gen with a bang, and led the way to CoD4 which I will let someone else talk about.

Wii Sports:  I didn't get it.  I am a Nintendo fan, and I was not looking forward to this.  I liked the idea of the Wii but the whole "blue ocean" thing seemed like the gimmick (not the controller, that always sounded cool to me.  And I knew the Wii would be successful).  When I bought the Wii, and my Dad and his wife, and all my employees, and my brother's in-laws, all made their Wii's and wouldn't let me play Zelda.   Who knew?  Well, no analysts and not many gamers (nintendo fans or not).  So everone I knew was playing... still are.  And you know what? It's great.  I love playing games and talking to people about games.  Now there are more people playing games.

Heavenly Sword:  I know, you're thinking, "What the hell?"   Well I thought the production values were some of the best in any game.  Not just in one scene, but the entire game.  It was short, but every moment was presented beautifully.   

Wii Sports: First time I used the Wiimote.

Heavenly Sword: As you said, it has great production values, and truly is a beautiful game.

Super Mario Galaxy: Talk about awesome gameplay! I love the physics in this game.

Uncharted: Drake's Fortune: Blew Heavenly Sword away in visuals, and had a great story throughout. This game has shown me what this gen is capable of.

Resident Evil 4: Wii Edition is the only one I can think of as being generation defining right now. The controls were so amazingly spot on, and so intuitive I was blown away. Everytime I thought I found something annoying about them it turned into something I loved after seeing how it worked in game. It cemented in my mind that the place for shooters is the Wii for me. While I do love the keyboard and mouse, and it is superior as a form of input, it just doesn't feel as good as a wiimote and nunchuck combo.

Wii Sports: ushered in the Wii phenomenon
Zelda Crossbow Training: Justified the Light gun aspects of Wii
Elebits: Showed the creativeness that could be had with Wii
Mario Galaxy: Showed how Wiimote can be used in interesting ways even in bettering current genres traditionally

Others to come eventually...

well since this game was first... besides gears which was awsome but..

bioshock- i was so danm impressed by the game, its graphics, sound, story, and everything. It pretty much amped up my expectations of how much effort should be put into games. Ive never been so into a game

Wii Sports/Play: Addictive and showed me how great the Wiimote idea truly is.

Super Mario Galaxy" Disappoint and makes me fear i will not enjoy a mario for yet another generation(sunshine sucked as well, last fav mario was 64) .

That's it for now, nothing amazing on PS3/360 besides graphics which i could careless about.

Bioshock showed me what a unique idea and brilliant artistic design can do to a genre (shooters) that was getting tired.

Mass Effect has redefined dialogue and story telling. It's simply incredible in this regard.

wii sports- A game where my grandma can beat me at bowling

Super mario galaxy- showed that the platform genre is still full of innovation

Elebits, this post-launch game really showed me what the Wiimote would do for core/hard-core games. While the graphics weren't great, the art-style was pretty cool. The controls and game design really shined. I love games with hi-scores and short levels, this brings that formula into the new generation with great controls and fresh ideas. Plus, the game was fun... just go get some coffee or something when the cut scenes start.

All of mine have been mentioned already: CoD2, Gears, Galaxy, RE4:Wii, Bioshock, WiiSports (the only video game I've ever been able to get my mother to play, by the way).

I think the real story here is just how many revolutionary gameplay experiences we've already had this gen and the oldest system is barely 2 yet