Buying a 60GB PS3 for about £350 back in 2007. If I'd just waited for another year or so, I could've gotten the superior Slim version (which I have now) for a much lower price, and had a bunch of cheap games along with it.
PS Move. It's actually awesome when it's programmed well in the game, and it's a lot of fun, but so few games actually use it and the majority of them are just shooters.
It would have been a lot of fun in Okami HD, but the controls are just ****ed up while using it, and it's so jerky that you need the blessings of Jesus, Allah, Buddah and every other God out there to draw a damn thing.
And of course, instead of improving or innovating, they just slowly let it die and hope people forget about it.