Ravious rightly points out that with all the free to play options open to MMO players these days, if you want to hang on to your subscribers you need to be giving them a steady stream of content. Give them a lump expansion and they'll burn through it all in a week and won't be paying you next month.
This is frankly what I assumed my subscription dollars would be buying when I first started playing MMO's. I see a steady stream of content becoming the standard way of operating for the MMO 2.0's of the near future.
It's amusing that the plethora of free to play games is going to force the improvement of subscription MMO's. I get the feeling that the "best of both worlds" (a steady stream of content without a subscription) is going to be Guild Wars 2, with the Guild Wars 1 War in Kryta event system acting as a test-bed.