Compared to EQ? Well, um...
1) Leveling is not purely endless grinding. To that effect...
2) Questing is meaningful. Vital, even.
3) Every class can solo. To level cap if they want, although it's easier to group.
4) Every class is useful to a group.
5) You don't NEED any class in a group. A group of 5 <insert your own combination> can do just fine with the right tactics.
6) The game seethes with energy. The graphics are more vibrant. The action is much more high-paced. There are aspects of the game, such as eating and drinking, that are designed to minimize downtime.
7) Tradeskills aren't total tedium, and even fun to do.
8) Every class gets new abilities every 2 levels. The non-caster classes get combat skills and such. And they are all vital to the game. No more "press A and snooze."
9) There's storyline to what you do. Every quest you attempt and complete has an explanation of why you're doing it, rather than just "bring me 4 gnoll paws as bounty and a few CB belts for good measure." It gives the game a sense of continuity and purpose.
10) It isn't made by Sony. That by itself is enough for many people to switch.
I could go on, but I think 10 reasons is good.
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Only the exceptions can be exceptional.