One thing I have been thinking about a lot as I play though, is comparing the game to WoW. Single player RPGs have a lot of things that MMOs haven't quite figured out how to duplicate yet. Storyline and lore is a big one. Blizzard's gotten a lot better at the whole idea with phased content and big events like the Wrathgate, but a single player RPG, with an audience of one, can shake up the status quo a lot easier.
You can start up a romance with that major NPC because hey, it's not like there's thousands of other player characters who would also like to sex up Jaina Proudmoore in a single player game. You can save the countryside from demons, and have it have a long term effect. Houses and people who you saved from destruction one play through may be gone the next because you failed to save them, and incurred permanent changes on the game world as a result. Again, phasing does somewhat address this idea, but in an MMO, it still creates an essentially homogeneous environment for all players.
One thing that I do love and that I do wonder if MMOs shouldn't try to duplicate is the in-depth conversation trees that a lot of single player RPGs use. Lately, Blizzard's been shunting a lot a of lore off into the comics and novels, and while I love a good novel, don't get me wrong, it does cause some disconnect with the approximately 9 million players who won't read said novel or comic book. Why should we be invested in characters who had so much of their character development completely off screen?
I'm a fan of show not tell, but all the same, being able to talk to more key lore NPCs and having a conversation tree that helps you get a grasp on their personality and beliefs and history would go a long way to making the game feel more immersive and getting everyone on a same page (or at least on the same book) lore-wise.
Of course, maybe single player RPGs could stand to learn a thing or two from MMOs. In Dragon Age and Mass Effect and other such single player RPGs, I'm struck at how generic all the armor usually is. There's exceptions, sure (mostly in plate armor), but I wouldn't mind a bit more variety. Then again, sometimes more variety just gets you the tacky fluorescent clown suits of the BC era, so hey, maybe it's not that bad.
Anyhow, I'm curious if anyone else has done their own comparisons of RPG types. What do you see as some of the key differences between MMOs and single player RPGs? What would you like to see either type do better, especially as it pertains to lessons learned from and by World of Warcraft?