First off, there's high-quality ammo, reagents, bandages, poisons, food, and water. Boring enough. In case you feel the need to look really bad-ass, or to get from place to place faster, you can grab Reins of the Black War Tiger/Horn of the Black War Wolf. All that is fine, but what about actual gear? All the Season 2 Merciless Gladiator gear is available, and all the Tier 5 class sets. There are also selected other items from the tier 5 raids (SSC and TK), as well as some badge loot to fill in the gaps. I'm even seeing the odd T4 raid item, like the Eye of Magtheridon and of Gruul, and some Heroic drops like the Scarab of the Infinite Cycle (these seem to be limited to the accessories).
Now that you've got all this awesome loot, you'll need some gems and enchants to put on it. Fortunately, there's quite an array to choose from. By my count, Blizzard has made 65 enchantments and inscriptions available, and 52 gems, including all the regular blue cuts, the cuts of Earthstorm and Skyfire diamonds, and several other gems obtained through drops and repuation. No epic gems.
And finally, there are hunter pets available in every species (as far as I can tell), from Bat to Wolf, so no matter what kind of pet you like, you'll find it at the tournament. Edit: except bears, apparently. Sorry, bear fans. Overall, just counting equippable items (armor, weapons, accessories), there are 441 pieces available (70 cloth, 68 leather, 70 mail, 76 plate, 73 weapons, and 84 accessories). But don't take my word for it -- head over and check it out for yourself. Did they forget to include anything? What items do you think are going to be overwhelmingly popular? If you're competing yourself, what will you be grabbing?
Update: as Killah noted, the S2/T5 on the tournament realms is a tier behind the S3/T6 currently available on the live realms. Blizzard clarified that this is their general intent in the tournament FAQ: "We intend to differentiate the gear used on the special tournament realm and the live realms and they will generally be at least one season behind."