I normally recommend DDO as I've spent a good amount of time playing it on two seperate periods in the past. Though it's had a recent big expansion that I've not played, so I couldn't tell you about the current state of the game.
Content wise, just be aware there's lots to pay for. Most of the content is available through adventure packs, which typically contain a wilderness area, a number of dungeons, and often a raid quest to finish off with. Some of the packs are absolutely shite (Threnal springs to mind), while others are much better (Vale of Twilight is among my favourite DLCs of all time). With only the free content you can expect to get to about level 15ish (regular cap is 20, though I think the new expansion increases that with epic levels), but that's going to involve a lot of repetition and a few bottlenecks (most paying players will use the adventure packs for levelling purposes, with stuff like Splinterskull, Co6, Gianthold and Sands being much more preferable to the free quests).
You'll also start off less powerful than other players. New players get 28 points for attributes at character creation while those with "Veteran" status unlocked get 32 points; you can unlock veteran status in game (which takes bloody ages) or simply by paying for it (one way around this is using the Drow race, which is very easy to unlock in game, as they only get 28 points even as a veteran due to their stat bonuses, but they're not such a great race choice for a lot of classes). There's also tomes available in the store, which provide permanent (though non-stacking) attribute bonuses. Most end-game builds will presume a 32-point build with at least +2 tomes all around. Again the tomes are available in game (they are semi-regular drops in endgame quests), but not always at the point in the game that you need them. For instance a +2 Int tome can be used from level 7, but doesn't become a regular drop until around level 15ish, and the bonus you get from it (an extra skill point per level) is not retroactive. Another common issue is from Dex tomes, which in many builds are required for feat prerequisites (the Two Weapon Fighting line in particular), and must be used before reaching a certain level in order to not screw up your character (while there are many ways to "fix" a character, they again rely on either rare drops or forking out some cash).
I'm probably doing a really bad job of promoting the game to you, but really it's a good bit of fun and highly under-rated IMO. There are some really good quests available when you get into it; the raids Against the Demon Queen (Zawabi's Revenge) from Demon Sands and The Thirteenth Eclipse (The Shroud) from Vale of Twilight are probably my favourite ever MMO experiences.
