Item level.
Theres a few pieces in the 29 twink bracket (quest rewards) that have item levels > 35, so they can get TBC enchants on them. For example, the engineering parachute cloak. You can get +12 agility on this. Or Pat Angle's fishing boots, you can get boar's speed or cat's swiftness.
Similiarly, you can be level 46 but you haven't updated your gear in a while. Maybe your bracers were bought off the auction house, and they're still level 32 bracers. You would not be able to put TBC enchants on those bracers, even though you're a level 46 character.
Edited, Sep 4th 2007 7:42pm by mikelolol