I'll go with what I remember 'cause I've been gaming for a long time!
NES : Super Mario Bros
SNES : Super Mario World and the super incredible graphics (at the time)
N64 : Super Mario 64 and the super incredible graphics (at the time)
PS1 : Final Fantasy VII (rented a PS1 and that game for a weekend and then I HAD to buy it)
PS2 : Bought it on release day with no game so I guess the system seller was my good experience with PS1 and the DVD player.
GC : Price drop to 130$ CAD with one game (Mario Sunshine)
Wii : Wii remote and its possibilities!
Future console :
PS3 : Lower price and/or MSG4 & FFXIII
Signature goes here!