One of the things that I love about World of Warcraft is that there's another block of new content available every few months. A lot of games that are popular in online multiplayer, like Diablo II and Counter-Strike, do this, but not to the degree of World of Warcraft, which will add new weapons, spells, dungeons, and battlegrounds from patch to patch. That makes sense, of course, since World of Warcraft is subscription-based.
I'd be interested in periodic mini-expansions for some single-player game I really liked, too. The only game that comes to mind that did this was Neverwinter Nights, and I don't know if that really ever caught on or not.