Bought Rift last night. I've put in about 6 hours so far. I never thought anything I would try would touch WoW. And Rift hasn't for me, at least not yet.
You are an Ascended, basically a hero brought back to life. Instead of talent trees like in WoW, where you pick a main spec, fill it with at least 31 talent points, and then can throw a few in the other two trees, you pick souls to bind to yourself. You can have three any any one time, but there are 8 in all for each class.
The Mage I started is warlock/necromancer/chloromancer. The warlock tree is much like in WoW, with DoT spells. Necromancer gives me a skeleton to dps for me and does a lot of life draining spells. Chloromancer is an off-heals soul, yes a mage that can heal. I've heard a spec deep in chloromancy can even be a mian heals in 5 mans with a skilled player. So not me, obviously

Other souls include dominator, a support/CC specialist, pyromancer, similar to a WoW fire mage, and elementalist, which does lightning and water based dps. Each class, Mage, Rogue, Warrior, and Cleric can fill more than one role. Clerics can do melée dps, think shaman in WoW, or heal (the main healing class), or even tank some things. Warrior is the main tank, but can dps and even do ranged spell dps with the riftblade soul. Rogues can be your average WoW stealthy assassin type, or also be hunterish with the marksman soul or a little of both with the ranger soul.
What makes it really cool is the namesake, rifts. Rifts can open anywhere at any time. They are level based, so a super hard rift won't open in a low level area, but then everyone in the area can participate. Looting is cool too in the rifts. If you enter a rift zone, you are automatically in the group. Loot isn't a need/greed system, but instead based on merit. Everyone who participates gets loot. If you just stand around not doing much, you don't get much. If you throw out leet dps or tank adds like a pro or throw out clutch heals, you get more and better loot.
There are multiple different kinds of rifts that spawn different mobs, like death for undead, water for water type creatures, etc. Rifts have to be closed. If they aren't, ie everyone ignores them, they grow. And grow. And grow. Eventually, if no one goes to them and defeats them, they can take over a zone, wiping out the towns and killing the npcs.
The npcs will respawn eventually, but it gives the game a real sense of community I think. It gives the game a little more immersion and makes it a little more social, instead of people just running around doing their quests. When a rift opens, a bunch of people mount up and head out. The graphics are also good, the rifts are like tears in the sky with tendrils coming out, and the view distance is good, so you can see the darkening in the sky from a good bit away. Tonight, two water rifts and a death rift opened in my area, it was insane. The while sky was black and you could see the tendrils coming out all around.
There are also invasions of the opposite faction, the Guardians or the Defiant (FTW!), where you have to repel the invasion forces. Similar to the rifts, they can and will take over a town leaving you SoL to turn in quests etc. The loot system in them works the same as in rifts, it's contribution based.
The rifts and the invasions are what I think could make this game great. The world is dynamic. It's not like in WoW where everything stays the same. Events happen everywhere, people work together.
Oh, also, people work together. You can ask questions in chat and get an answer. I'm sure CMD and blue can attest, there are a lot of immature, stupid jerks in WoW. Rift seems to have a more mature player base. I guess that's due to just how many people play WoW, there's going to be more trolls than a lesser known/played game.
No, I haven't canceled my WoW sub, and don't plan to yet. I'm not in love with Rift like I am with WoW. But as I progress - still have yet to sample an instance - I could see myself playing Rift much more than I've played WoW recently. After all, WoW has been around for what, 7 plus years now? Eventually, there is going to have to be a successor to the most widely played MMORPG in the world.
*Please excuse any spelling/grammar/autocorrect errors, I composed this on my iPhone.