good question.
all MMORPG's have the same sort of structure; questing, items, lvling, that sort of stuff. so at it's core, all MMORPG's are essentially the same, it's how they take that core and tweak it and execute it that makes MMORPG's different.
with that being said, LOTRO isnt much different than WoW really. what i find enjoyable about it is first, the quests. many of the quests well thought out, and fun to do. also, i like the scale so far. you go to Bree, and it's a very large town, or go to Archet and it's a tiny little town, but you can believe that all the npc's hanging out there actually live there.
i also like that LOTRO tries to encourage grouping, or fellowshipping as it's known. in WoW, i hated grouping with people i didnt know, because of smacktards and flakes, it wasnt fun for me. in LOTRO, i've jumped into a fair number of fellowships, and most have been enjoyable. i have a feeling it has to do with LOTRO having a much older player base then WoW.
i like the graphics, the quests, the fellowship, the lore, and the fact that i can solo or group and get stuff accomplished either way. for me, i've enjoyed LOTRO alot because i've decided to approach it from a more casual gamer perspective.
what isnt so fun about the game to me is i dont care for all the classes, and there not many of them. i do like the crafting system, but it's a bit tedious, and there's lots of running around till you get a mount at lvl 35. i have also found that, even tho i really like the game, i find myself getting slightly bored if i play for too long. more than likely thats because i have gamers ADD!
well, i hoped that helped some. i too cant wait for Warhammer Online!