Great for casual and serious online players
There's no crazy million hours spent grinding. It's level, reward, mission, accomplishment for the story / mission path goers. For the PvP groups it's skill , counter, heal, support and fun fun fun. I haven't seen a more balanced team PvP system and recommend this game for those that enjoy PvP.
The only problem I have ever had (and rarely at that) is to find a good group for the harder co-operative missions. If you're patient enough one group will form and work out in the end. Guild Wars is not like the templated MMO's out there and is missing somethings like really detailed crafting or merchant careers but the rest of the game holds up despite this minor missing element. When casual and serious players ask me which MMO or just online game to get that isn't FPS I point them towards Guild Wars.