I think RTS games are less addictive because they are broken up into discrete games and are much more intense in terms of mental energy and frantic clicking. Whenever I play a good game of StarCraft or WarCraft III, I usually feel really drained and don't feel at all like playing another game.
I agree. It seems like you never get enough oomph from an engrossing session of WoW - never really satisified, always feeling empty after the latest chapter/quest.
It's kind of like reading a Robert Kiyosaki book - getting to the end and needing to buy the next book right away. The content makes you feel good but it always leaves you feeling a bit undernourished, hoping the next fix will finally complete you - leaving you whole.
The preceding was a bit of self-reflection, perhaps. :)