A lifetime discount on Store purchases would be great
And some truly unique decal or vehicle skin, which will not be available to all players in the Cash Shop after 6 months... I'm reminded of the sunglasses that were in the Starwars Galaxies CE, which were totally unique in the game. You could spot a CE player a mile away. For the first 6 months anyway, then they were released to the general player base, because obviously they were just too "cool".
The goodies in the average MMO "Collector's Edition" or "Founder Packs" are usually only "exclusive" for the first few months (or even weeks) of game play. It would be really nice to have some cosmetic (and visible in game) perk that will remain that way for ever.