I played some DDO but just a little bit of LOTRO, mostly before it went F2P, but from what I remember they have different style, with DDO being a lot more instance based, where you have a multiplayer hub in the city, but when you do quests you won't encounter players you haven't specifically grouped with, sort of like with Guild Wars.
As a single player RPG fan DDO felt good to me, because of the structured quests: well designed maps, a story, scripted encounters, barrels to smash to find loot, everything I expect from a D&D RPG.
The main problem with DDO is that unlike most FP2 MMO's it's gating content. What you pay for are adventure packs, and you quickly run out of free things to play as you go up in levels. Most other games have other restrictions (which DDO has too) but let you play most of the content for free.