Not being an avid raving fanboy hopefully I can give a relatively unbiased opinion:
I've played WoW, Guild Wars, Anarchy Online and a variety of MMORPGs. I bought this yesterday and playing just the first few levels up to 10 (which are almost always the worst) I've easily enjoyed this game most out of all the MMOs I've played. Also bear in mind this is a game only *just* released, MMOs add an awful lot more over time, so this is set to get even better.
The game is, so far, entirely quest based with no or little grinding, a far higher proportion of your time in the game will be spent playing against other players in PVP and doing 'public quests' (where anyone can turn up and help out in one huge quest, the rewards are shared out based mostly on effort and in 'influence' which can earn you rewards) which I think is a great thing. Also I find the whole game a much slicker experience, with less wandering around lost, less time spent searching hopelessly for a party, being argued with by a kid.
There are also several forms of 'experience' within the game, the standard type which raises your levels, but also the aforementioned 'influence' from public quests which allows you to raise stats, and 'renown' which allows you to purchase better items with your gold.
The reason I've left the review at 4 stars is that whilst its the most fun I've had in an MMO I still don't feel it's a perfected experience: theres a slight lack of character differentiation especially at my current level, and *still* I am often stuck doing menial messengerboy tasks that all MMOs force upon you. This game doesn't redefine the genre into something amazing, but it works very well within its traditions.