Check out the BFF reports on Rifts to see some of the different things about it.
As for comparrison's to WoW, almost all the UI commands are the same, the basic layout is similar enough that wow players can easily figure things out without turning on Tips.
The biggest difference is the Soul system. 4 classes, 9 souls each, and I think you can have up to 4 "roles" (like dual speccing, but 4 times instead of 2!). You basically pick your armor class, (Warrior - Plate, Cleric - Chain, Rogue - Leather, Mage - Cloth), and each of those AC's give you a bunch of souls that other games would use as completely different classes (I.E. Mage - Cloth - Warlock, Mage, Priest; Rogue - Leather - Hunter, Rogue, etc.)
You get to mix and match three souls actively per toon, which is cool. I play a Pyromancer/Warlock/Archon (basically a Fire spec mage with warlock type dots, and archon as support for buffs and debuffs and mana regen).
I'm really enjoying it!