PS3 -- It's odd, but the first factor is that I quite WoW. I was a 100% PC gamer after the PS2, mostly MMOs. My best friend and MMO partner passed away during that time, however, which took the heart of playing WoW right out of me. It just wasn't the same anymore. A few months later, though, I watched an E3 for the first time in years and got blown away by Uncharted 2. The press was absolutely raving about it. That, and the teaser for The Last Guardian are what made up my mind to get a PS3, then the deal they ran where you get $100 off if you get a Sony credit card clinched it.
The funny part is, obviously, that TLG never showed up, but also that I turned out not to be a fan of Uncharted. All that was completely fine, however, as Valkyria Chronicles was one of the first games I got and I absolutely fell in love with it.
Vita -- Mostly Persona 4 Golden, to be honest, as well as word that other RPGs were coming to the system.
PS1 -- Final Fantasy VII commercial. I still remember seeing it for the first time. It was one of those cliche moments were your friend is talking and you completely tune them out because you're sitting there in awe.
SNES -- A rich kid offered to trade it to me for some skateboard parts. In the deal was a Mario game, NBA Jam, and A Link to the Past. Pretty sweet deal. I was like, "are you SURE about this," but he just said his parents would buy him another for Christmas if he wanted it.
