Final Fantasy Versus XIII will feature party-based combat, Magic, Item, Summon, and EX-ARTS, as well as "a realtime event system that seamlessly transitions attacks and other gubbins into the flow of gameplay." Nomura asserts that this new system is taking a great deal of time to develop.
Though more specific reasons for the delay aren't revealed, the decision to do so apparently stems from Square Enix's desire to do "a number of incredible things" with the game. Whatever those "incredible things" turn out to be, he explained that development time on FF XIII Versus would exceed the time spent on other Final Fantasy titles like Type-O and XIII-2, which are both planned for 2011 releases. Nomura further clarified that the next official update about Final Fantasy Versus XIII wouldn't happen until this year's Electronic Entertainment Expo (E3), which makes it reasonable to surmise the earliest possible release of the game is sometime in 2012.
I bet they just don't want to release all 3 games so close together so they aren't competing against each other.