The Feel of RIFT is very very simaler to WoW, but don;t let that fool you, this one a very different game. The two biggest differences is RIFT's Class system, and the "Rifts" themselves. RIFT only have
4 Classes, Warrior, Mage, Rouge and Cleric. While this may put you off at first, each class has 8 "Souls" (WoW equivalent of your Spec), and you can use three Souls at one time, and all at once. It Takes some getting use to, but it add near limitless Class customization and some real flavor to the Game.
Next is the "Rifts" these are basically Out Door raids that will pop up in the different zones. They are a very Important part of the game as well, because most of the time, the Monsters that come through the Rifts are much stronger than Zone monsters, and if the Players do no come together to close the Rifts, they will soon overrun the Zones, killing NPC and even taking over Cities and Towns. Rifts start from the get-go and are in just about every Zone, you can get some really nice stuff from them too with a little luck (though mostly you'll just get Plainite (think WoW instance points [JP, VP ect ect ec]) and some one use Damage dealing items.
This might seem like a nice part of the game, but the problem is that the Rifts hit so often, that it can get very annoying, and as stated above, if they are not dealt with, then they can take over a Zone fast..
All in All, RIFT is not a WoW Clone, although it's absolutely got the WoW Flavor to it (somethings, like the Achievement system, can be plucked right outa WoW). If you like World of Warcraft, then RIFT is definitely something to try, but it's not so similar that you would get tired with it fast.