I spent several hours in the third Rift beta event, playing two characters of the two different factions. While the guys from Trion are most proud of the "rift" system they named the game after, most MMORPG veterans remarked that rifts work pretty much exactly like public quests in WAR. Thus I'm going to talk about another innovation of Rift, the soul system.
Rift has 4 basic classes, warrior, rogue, priest, and mage. But each of this classes has 8 sub-classes, called souls, and players can freely choose 3 of them. Each soul has a talent tree, and in addition to the talents you get spells and abilities based on how many points you put into each soul. The overall effect is similar to being able to choose 3 talent trees out of 8 for your class, instead of your class choice determining which 3 talent trees you get. Furthermore you can buy souls and respecs, so while your basic class will remain the same, you can switch between the 8 possible souls and redistribute your points.
This gives you a lot of interesting strategic choices. You can choose souls where the talents of one contribute to the performance of the other, or you can choose souls that do very different things, like adding a necromancer's skeleton pet to your pyromancer fireball-hurling mage.
Rift has 50 levels, and you get 1 soul point per level, plus another point every third level, for an overall 66 points. You can only put as many points into a soul as you have level, so there are always some points to put into your second or third soul. Nevertheless, like every talent tree in every game, the best talents are at the very top, and I expect to see many 50/16/0 builds or variations thereof among minmaxing players. So while the different souls contribute a lot to the early game, where just choosing a soul gives you additional spells, in the long run the best tactical choice is to make your main soul dominant. Maybe Trion should give out slightly more soul points, e.g. 75 at level 50, to combat this.
The harder task for Trion is how to balance that all. The nearly forgotten term "tank mage" from the UO days springs to mind, describing people finding a template that is able to do everything, without having any weaknesses. Of course the beta was too early to say how balanced the classes and builds are. But fundamentally any system that allows meaningful choice also allows players to gimp themselves, or find the "flavor of the month" "best" template, until some nerf in the next patch changes everything.