I purchased the first game, Prophecies, the day it was released, and I still hop on for a bit every week. I own all of the expansions, too. It's a very fun game. As Hellriser mentioned, it takes a different approach than most MMOs. Eight-slot skill bar for yourself, and you are allowed to carry up to 7 heroes (AI controlled henchmen) with you into instances, or "explorable areas", providing you have the expansion(s) for them. You can customize your heroes with any 8 skills you choose, excluding the PvE-only skills, which are reserved for players. You are also able to carry up to 7 other real players (11 in some areas) instead of your heroes into those instances.
Unlike most MMOs, players are met in towns and outposts, not aimlessly wandering the map. I haven't been too up to date with GW2, but as far as I've heard, it will be an open world, with no instances. Looking forward to it