Loot

One of the things about the World of Warcraft loot system that I think is particularly fantastic is that even if you get unlucky and the monsters don't drop the wonderful shiny things that you were hoping for, eventually, you can still get some sweet stuff.  It's sort of like a consolation prize.  It's a great mix between that exhilarating feeling you get from RPGs where you equip something awesome and powerful and new that just dropped, and a mitigation of that obnoxious feeling you get from early versions of World of Warcraft (and many other RPGs) where you've been playing for days or weeks and haven't gotten any new gear.  This is particularly important in WoW because much of WoW is played at a level cap, where you progress mostly only through acquiring better equipment, as opposed to most other RPGs where either there isn't a level cap, or it isn't practical or important for most players.  (Diablo II's is 99.  You can beat the game at around level 30.  Only the truly hardcore minority ever get to 99.)

Bosses in current level 80 dungeons and raids in WoW drop an item called Emblem of Triumph (or a better version, the Emblem of Frost).  Once you collect enough of those babies, you can turn them in to a curator of artifacts in the main city of Dalaran for a piece of powerful new gear.  As long as you kill enough bosses, you're guaranteed to eventually get something good from it.  If you're lucky, along the way, you got a few other upgrades to your inventory too, but at the very least, you know that after a few more bosses you'll have enough Emblems to buy that shiny new Brimstone Igniter or Bloodshed Band.  It's something to look forward to, and it's a powerful motivation to keep playing if you've been really unlucky in your past few expeditions.

In contrast, years ago, before World of Warcraft's first expansion, at some point I would play for weeks to a month between getting an upgrade to my equipment.  Since upgrading your equipment is how you get better (other than your own player skill) when you're at the maximum level and XP the game allows, things become stagnant.  Multiple times I considered that it was time to quit.  (I would have, but I decided to try out another couple classes first with one of my friends.)  I haven't felt any desire to quit for a long time.  Most single-player RPGs don't have as much of a problem with this because you just keep leveling until the end of the game, but WoW doesn't end, so it has to have an artificial cap somewhere.  Blizzard has discovered how to bring back that feeling of constant excitement of improving your character even though it's currently impossible to level from 80, where most players are, to 81.

The stars have aligned and I've been saving up—I'm getting six new pieces of top-quality equipment within the next few days.  If I'm lucky, I'll get some more from the bosses I kill.  But even if I don't, I know there's a reason to keep playing my 30-45 minutes every night.