Pursuant to my earlier post on WoW, I think I've been able to discern why it's the king of the MMO's, which is something I wondered about for a long time. Basically, it boils down to that there's so much to it, that there's something for everyone.
Want to solo all the time? You can do that in WoW. Want to run dungeons from beginning to end? You can do that in WoW. Want to PvP? You can do that in WoW. Go through life never once pwning a n00b? You can do that in WoW. Geek out on the world lore? Yup. Ignore the world lore and just kill monsters/take their stuff? Go for it.
LotRO, by comparison, bounces back and forth from focus to focus, and from philosophy to philosophy, often forcing you into a playstyle you don't care for into order to slog your way through to the next bit that you like. It was originally designed with the idea that you'd be grouped most of the time (because FELLOWSHIP!), but then players kept wanting to solo everything so they had to retool at least the main story, but then raider types kept complaining that the game was "too easy" so they added tiered instances, blah-blah-blah-- and different areas/levels of the game, retooled or no, still reflect all this back-and-forthing. WoW seems to easily support lots of different playstyles all at once.
I don't know if WoW was always this way, or if it's a reflection of how long it's been in operation that it's reached this level of gameplay sophistication. But I'm impressed with how well they've managed to keep "all solo all the time" (which is mostly how I play) interesting over 60 levels. There are recurring patterns of course... "Go kill 10 _____'s. Now go back and collect 15 ______'s. Now go back and kill sub-boss. Now go back and do quest line capstone." And in the case of my hunter, she pretty much only ever uses four skills. But the locations have been varied and imaginative enough, and the stories behind these quests interesting enough, that it rarely feels like a "rinse-and-repeat" slog. The ability to tame landscape creatures and have them as companions makes it fun to find and try to catch uniques, and the battle pets make a nice mini-game to break up the monotony as well. WoW has showed me some things I never saw in fantasy before, and given how much fantasy I've read (and fantasy gaming I've consumed), that's a pretty impressive feat.
The dudebro problem I mentioned before is still there, but happily it's fairly easy to tune it out. Stay out of "General" chat, find a Guild full of cool people, and you'll probably be all right. Just don't ever use the dance emote with your night elf. ¬.¬