All of the game's champions will be available right away in Dominion ("That was the goal from the beginning," says lead designer Richard Hough), with all of the same spells they'll have in Summoner's Rift. The responsibility for balance, then, will fall mostly on the game's items -- some items aren't available to play with in Dominion, and vice versa.
Madred's Razor, for example, is one popular item that's locked out for Dominion. "There's much less of a focus on the laning phase in Dominion," says George, "and furthermore the concept of how the jungle works is much different. There aren't really neutral creeps, where Madred's is really helpful." One of the new weapons is called Ionic Spark. It adds health and attack speed, as well as a proc that does a chain lightning attack to a few nearby enemies.
I suspect extremely late-game carry items that require charges to build are also going to be absent. That means we probably won't see:
Rod of Ages
Tear of the Goddess
Boots of Mobility will probably be removed for balance reasons.
These are just the ones that come most readily to mind.
Riot should have/should still rebalance the item shop in Twisted Treeline, too.