I personally can play games like Neverwinter Nights and Baldur's Gate for hours nonstop, and its huge fun for me.
Neverwinter Nights almost rules even more when played in Multiplayer, so its really not Singleplayer vs. Multiplayer being the issue here.
I think, MMORPGs just boil down to "look I'm so strong" and mindless grinding for me. In classic RPGs, your characters grows more and more powerful, and I sit there thinking "Wow, remember how fragile your character was in the beginning and what a long way he has come?".
When I played WoW for a while (Test account), I felt like: "Oh, great. I'm going to waste a shitload of time in order to reach max level. And that is not even special. Look at all those max level guys."
I really have the feeling that a "max level character" should be like... "OH MY GOD, THE LEGEND IS HERE!" and superstar-like status in the game's world.
If a game could somehow limit the number of those characters without feeling too restraining, that would probably drastically improve my view of those games. If somehow the story would be presented in a similarily breathtaking way as in the best Single Player RPGs and the quests wouldn't boil down to a ridiculous amount of grinding, maybe I could even give those games a shot.
Sorry.

(About that limiting thing: Maybe a political/event driven thing could help there, like, "only people that are higher in the hierarchy than this/that level may get past level 50 or something along those lines, or I dunno what...)