Dungeons and Dragons Online, sorry.
Sort of based on the D&D 3.5 edition ruleset, set in the Eberron campaign settings. Gameplay is centered around instanced dungeons rather than just wandering around killing things, though there are a few instanced outdoor areas too for that sort of thing.
Quite a lot of build variety is possible, though it can be unforgiving of mistakes - they usually require time or money (either RL or in-game) to fix.
Level cap is 20, and you can get to 10 easily enough just on free content. Extra content is sold in themed adventure packs and unlocked for your entire account, additionally, people who already own the content can buy you timed guest passes to it.
I've got a couple of videos of it up here, including a complete play through of one of the adventure packs, which starts here:
Sound is pretty rubbish until you get to Chapter 3 - was my first set of Let's Play videos
And a trimmed down playthrough of one of the free adventures here:
This playthrough isn't the full adventure - the focus of the video was doing a trap heavy quest with no rogues.