economy is perhaps the least understood. Where damage over time and powers can be easily tweaked to bring them in line with the rest of the game, virtual economies can bounce around more than a hyperactive kid with a super ball. Even if you think you can find a "fix" or a "tweak" to bring your virtual economy to where you think it should be, the economy can throw you a curve ball and react completely differently.
Over at the Warcry Network, Jonathan "Ithelsa" Steinhauer has pitched the concept of going back to a barter economy, similar to the one that Asheron's Call accidentally stumbled upon. Instead of relying on money sinks and economic controls, the barter economy places the value onto items and craftable materials -- thus providing wealth that can easily increase or decrease over time and naturally sink out of the system as they are incorporated into other items.
The full article is worth a read, especially if you're interested in the tricky issue of virtual economies, or just general game design of virtual worlds.