To the OP.
WoW is still considered to be a dual-threaded app based on how effective it's threads function. Current Operating Systems simply spread the load across all cores to lower the chance any will be maxed out to allow smoother multitasking without worrying about assigning cpu affinities. There is a 3rd thread but it's not a real thread as it's not effectively used, it's a minor-at-best "thread' and doesn't qualify as a true thread, but for that very reason, some people based on their hardware configuration will use a different affinity setup in WoW's "W-T-F" as it sometimes does help them but it's always been with the much older quad cores or the tri core procs. It's important you know the improvement isn't significant by any means and normally only helped towns to load faster. There is a fourth thread, but it's basically not even worth mentioning as it's effective usage is somewhere around say 1% of everything else.
WoW is extremely CPU dependent as more people appear around you, this is why your framerate drops significantly around others, especially in raids and towns, enabling ultra shadows makes it extremely more taxing. This is also why you can be in a town with near max CPU usage and only 50% GPU usage.
As long as you don't have a video card with a low amount of Vram, the most taxing setting is Ultra Shadows, in my 8 years of experience across many combinations of hardware, going from Ultra Shadows to down the notch below it on average gave me 20 fps when in a town as Ultra Shadows around that many people ate up CPU cycles like they were candy.
MoP's new shadow setting "SSAO" shouldn't be too demanding, but I haven't had many chances to examine it as I'm retired from the game, but I did mess around for a few days post MoP client patch. But, if it's adding more shadows then logically it's going to be even more taxing, I just can't say how much.
I'm also aware there's an even further view distance now and much higher quality textures, but I can't say much of anything about them.
Also, if you haven't updated your mods, there's a very high chance one of them is significantly hindering your framerate, it's very common.
If you ever see yourself in a situation where you target something or enter combat and your framerate drastically decreases, that's an old/bad combination of settings relating to mods, then you have to wipe your Interface/W-T-F folder.
Edited by Ascii Aficionado - 10/4/12 at 2:34am