Founders (that is, players who had purchased a boxed or digital copy of the game) will have access to eight character slots, a special title, and a special mount. Subscribers who had not previously purchased the game will be limited to two slots with more purchasable. Of course, subscribers also get shortened dungeon cooldowns, more auction house listings, and several bonus daily items. Looking to get in on the Founder benefits? You'll have to move quickly; digital sales of the game will be discontinued upon free-to-play launch, and no further boxes will be produced, so you'll want to buy soon to secure your benefits.
[Thanks to Justin for the tip!]