While Warcraft still remains the focus of my end-game gaming, I've been messing around in Rift since it's launch and It's a very enjoyable game much akin to Warcraft. It is, in fact, a Warcraft clone, which is a good thing, as it'd have little chance of surviving if it wasn't.

How similar are these two games?
The menu system, action bars, default layout, professions, zone progression, reputations, dual-factions, battlegrounds, instance group size, default key bindings and combat system are all strikingly similar - down to where to change the setting to show your helm, how to pull up your character or profession window, etc... It is a brainless transition for those searching for a Warcraft replacement.

What is different in Rift?

Der... Rifts. They are both awesome and annoying. They show up as little swirly things on your map and upon mouseover, you can see if they're minor (relatively soloable) or major (almost definitely not soloable at level) rifts. Also on your map, you can see the paths the invaders are taking during an invasion. These are very cool and different from anything we see in normal Warcraft play and will indeed take a day or so to get used to. Tips: If there is a major invasion happening in the area you're trying to quest in, either go do rifts or pvp for a while or be prepared to die miserably as you accidentally pull 8 elite mobs you didn't see coming. Rift rewards are based on participation regardless of the group you're in and participation is based on how many global cooldowns you use (heresay), so buff people and use your instants if you're out for max rewards. Don't bite off more than you can chew - as in normal play, pull one at a time until you can get a feel for how hard they hit and how easy they are to kill. Join the public group so you can heal other people, be easily healed by other people, and so you can feel more productive not always seeing grey health bars (meaning it's not your kill).

Aggro Radius - it changes very slowly and mobs can knock you off your mount while you're trying to get to your destination, which is mildly annoying until you realize most mobs are on a pretty short leash and thus reset after following you only a short distance. It's still an adjustment to make coming from Warcraft - land of easy travel.

Level Scaling - Fighting things above your level is dangerous business. 2 level 17's should be able to kill a level 20 elite, right? Fuck no. You will die in 3 seconds flat unless you happen to have a good tank pet and a healer (the holy trinity), but even then, you're pretty likely to eat it. The more you play, the easier taking on multiple mobs and elites will get, but when you first start out, you're likely to be shocked at how squishy you are - I know I was.

PVP - will take much longer to formulate sound plans of attack given the amount of choices people have. You see rogue, but it could be a hunter, a WoW-like rogue, a Bard, all 3... You see a Mage, but it could just as easily be a healer. You see a cleric, but it might be melee or instant-cast heals or cast-time, etc... I've always been easily confused when PVPing, but this game takes it to new levels of "WTF was that?" There are designated PVP trees (souls in Rift) that you can purchase after about 4 hours of running battlegrounds. How many points you can put in these souls depends on your PVP rank, so you'll likely just be replacing your 0-point soul with this one at the start, but I'm guessing it's worth it. For my cleric, the 0-point talent gives me "break free," an ability that gets me out of fear, stuns, etc, which is awesome.

Healing - not just 4 healing trees. Rogues and mages have viable healing abilities which make them very useful in instance groups and PVP. Holy shit can they pump out a LOT of group heals! These souls also make solo play a bit more friendly, as your survivability goes way up. Within the main healing class (calling in Rift), the Cleric, there are a veritable shitton of different options for every healing style. I LOVE this! you want to be a druid today - dooo eeet. Shaman tomorrow? yes please! Paladin? Priest? gotcha covered. It's a beautiful thing - perhaps the most wonderful thing in Rift thus far. I started out as a Sentinel (very pally-like) and have since moved to Warden (shammy/druid ish). There is some overlap in the soul talents, so if you're looking to be a healer, pick souls that complement each other well - your main soul, then one with 10-20-point talents that complement nicely, then a 0-point soul that you can switch out whenever you like without paying to respec.

Beauty:
Character creation is marginally more interesting than WoW and your toons look much better overall. It's not as customizable as I've seen with no ability to change your body style and still too few hair options.
The world itself is very well done - not as pretty as Aion was, but the textures and lighting are beautiful - truly dynamic and realistic.

Leveling:
There aren't any leveling zone choices in Rift - you start here, then go there, then there, then there - it's very linear. If you don't like a zone, too freakin bad - do it anyway. The only other alternatives are PVPing or Rifting to level, which you can do, but it takes arguably longer unless you win every match or stage with ease. Overall, leveling seems to take less time per level than in WoW, but Tirion has stated that the bulk of the game exists at max level, so we shall see about that.

Instances:
The first 2 instances are very easy to get through - nothing spectacular to look at, no interesting boss mechanics. The bridges and walkways in Deepstrike Mines were cool, but not game-making. So far, I can easily say Warcraft instances are more fun, but Rift's instances could definitely improve as more class distinction emerges and more instances are explored. I'll give it time before my final evaluation.

Screenshots (mostly just me trying to capture the amazing lighting effects, which is really not possible - you just have to fire it up and go stand by a fire to see for yourself):