I enjoy DDO as much as any other player, but the biggest problem I have found, is that there is a severe lack of solo content past the harbour. While I realize that DDO is very much focusing in group dungeon delving... Many people I know have opted instead to play other games (Such as WoW or LoTRO) because of a distinct lack of solo player content past level three.
While group play should continue to be encouraged, and strongly reinforced with large raids and raid loot, there needs to be more solo player content to allow players without as much time as others, to play alone and still gain levels past the third. This would not only increase the player base, but allow players like myself with heavy work schedules to get further in the game, and make it worth the monthly payments.
While this is only my personal oppinion, I know at least five of my friends who have quit DDO for this very reason. Please consider this when looking to develop the game further.