Fun classes, vast environments, war and fun pvp. Very ambitious design. All the ingredients are there...
From level 1 the game has been fun. I haven't felt weak ever, even when in RvR scenarios. I am currently at level 13, and I am hooked.
The RvR (pvp) is superior to WoW, hands down. Larger bases, more varied terrain. You also won't get 2-shotted by someone with superior gear. There are mechanics in place to keep high level players from ganking low-level public RvR. I won't ruin the surprise.
You gain experience from PvE and RvR both.
The maps are varied and fun to explore. It is fairly easy to travel to distant areas, regardless of your level.
The graphics are great and will run smoothly on most mid-range computers, ensuring a larger fanbase. If next-gen graphics are really important to you, this may not be your game.) If you are a WoW player this will be a huge upgrade.
The art style is very well done. The opening cinematic alone was almost worth the $50. It made me want to jump right in and start playing.
The Achilles heel of this game is the need of a sizable player base for the best parts to work. If no one is online, you are stuck doing pve. If there are lots of people online, there are a lot of areas for potential RvR (pvp) and the game becomes that much more fun.
I didn't expect I would like WAR but I do. I hope it catches on, because there is huge potential here.
UPDATE: I have lowered my score by a half point, because 3 months later there are still a lot of bugs, and very few announcements from Mythic regarding updates/fixes.
UPDATE #2: The game has undergone some changes since my last review. Bugs are fixed and the game runs without any problems. The subscriber base has shrunk, but there are fewer servers now, so they feel quite densely populated. I have come back to WAR after trying Aion and Champions Online, and I think it has a lot going for it and is still relevant. The player vs. player is really a blast.