My favorite of all the available MMOs after playing these games for about five years now. Much closer to pencil and paper D&D than Neverwinter Online and a much better game as well.
* Extensive content after eight years of updates and expansions (since 2006) with two major D&D campaigns: Eberron and the Forgotten Realms.
* The original quests are focused on group play and are similar in layout and tone to pencil and paper D&D modules.
* More detailed character customization than any MMO that I know of. You can play a halfing ninja assassin (dark monk), a drow elf wizard / fighter, or a warforged artificer / ranger, among many other combinations.
* True Reincarnation system which allows a single character to start again at level one with a new class and/or race and become more powerful each time. Indepth replay ability for this reason.
* A mature, dedicated player community that wants to expand the community.
* Ongoing bugs / exploits that are being slowly addressed by Turbine - but this doesn't make the game unplayable.
* The graphics and some other features are aging when compared to very new MMOs.