graphics didnt do it for me.... it was playing dead rising in a demo ... i think at a compusa.... the game was just so fun, and i hard spare cash at the time picked up a system and 3 games deadrising, doa4, blazing angles (im a sucker for flying games)
that was summer 06 i think

and if you want older gen stuff..... i didnt think i was going to get into the ps1/n64 era, then i played FFVII at a friends house... changed my mind on 3d gaming  (mario64 demos had turned me off of the whole idea 3d could be cool)

I bought every console I've ever owned on the date that a specific game released because they were awesome looking.

NES = Super Mario Bros. 3 (yep, i played 3 first)
Genesis = After Burner 2
Playstation 1 = Metal Gear Solid
Playstation 2 = Metal Gear Solid 2
Playstation 3 = Metal Gear Solid 4

MGS4 for PS3.
Legend of Zelda Twilight Prncess for Wii
MAss effect for Xbox 360