First, let's talk about the amount of content that an MMORPG has. MMORPGs in general have a ton of content. Quest content, things to do, places to see. Let's look at one MMORPG that I'm playing right now, Lord of the Rings Online. This game has a grand total of 21 huge zones which you can explore, with well over 2000 (!) quests to be found in these zones. Not only is that a ridiculous amount of content, more content is added all the time through expansions (of which there are 4 of at the moment) and updates. That's enough content to keep any RPG fan busy for a long time.
Exposition of lore. One thing I find great about RPGs is how they build a coherent game world using fantasy lore. In a game like Lord of the Rings Online, the lore is expounded upon to a very high degree. Every quest you find is weaving some small story in the world of Middle Earth, and you are learning more and more about the world you're in with every task that you complete. This also goes hand in hand with the amount of content. Since there are more quests around, there is more chance to expand upon the lore in every situation. Being a lore junkie myself, this aspect of MMORPGs is most satisfying. Single-player games have this as well, but usually the worlds are not quite as filled out as the MMORPG counterparts, mainly due to the sheer scope and size of an MMORPG being greater than that of a single-player RPG.