For all you non-programmers - multi-core/multi-processor systems have more than 100% CPU available. Each core is 100%. My iMac with Intel Core i7 is a quad-core processor chip, and also has "hyper-threading". Hyper-threading appears to the OS as if each core is 2 processors, so in effect I have 800% CPU available.
Most of the things you do on your computer probably take no more than 5-10% of a processor, so if WoW is taking up 70% of your total processing power, you'll still have plenty left to do things like open a web page, edit a document, or even watch a video. Most of the time you're loading a web page, for example, you're waiting for network traffic.
However, many systems, especially laptops, are not really designed to be run at such high CPU utilization for long periods of time, the extra capacity is so it can finish off things like re-sizing a photo quickly so you don't have to wait. The speed of processors these days is really amazing, most systems are way overkill for what most people ever use them for.
That's why a thread that's using up an entire OS-visible processor (i.e. 1 of the 8 on my hyper-threaded system) can be such a problem, since it is ALWAYS running. Most CPU processes are waiting on things like network or disk data, the CPU goes idle, required power drops, heat produced drops.
When running with limited FPS (including vertical sync), the CPU should simply be going almost idle between frames; even with unlimited FPS, it will still be spending a fair bit of time waiting on the graphics processor (higher graphics settings can actually lead to LOWER CPU usage, but higher GPU utilization).
BTW, I've verified that it's a single thread that's chewing up all the CPU time, but haven't had time to go in and see what it's up to.