I found an interesting read on "Solo" players in MMORPGs. I fall into this category. I pretty much agree 100% with this article. I like the ever changing aspects of MMOs. I like the lore and stories of WoW. This is probably why I pick up WoW everytime an xpac comes out I suppose. I like to be able to play with a few friends if I want. I don't like to commit decades of time to scheduling raids 4 nights a week. WOTLK looks like it will be easy enough to pick up a group for the 10man dungeons. I love the large economy's and interactions. I sell something, a real person bought it. It's not static. Blizzard has done well to get back to it's roots and add some content for the Gamer with a job like mii.
In the same regard I love sinking hours n hours into a nice, well crafted Solo RPG like Final Fantasy I, Avernum, Eschalon, etc... You are the hero, the one, the savior.
Give it a read.