If, like me, you're running WoW on a system that hasn't go the latest greatest graphics card, you've probably noticed a drop in performance with the new expansion, so here's a few tips to get it back up (at least a bit);
1 - Defrag your hardrive, you've just installed god knows how many GB worth of stuff, your files are going to be all over the place. This might not help much, but every bit counts.
2 - Turn down your graphics settings, particularly your viewing distance. This is one of the biggest ways to improve your performance, and it doesn't change the experience much. beware though, I've already found 1 or 2 quests that involve looking a long way into the distance, and are impossible on the lowest settings... shame on you Blizz.
3 - Check this:
http://www.wowwiki.com/ExtShadowQuality apparently Blizz have brought in a new shadow system, which looks nice, but drops your FPS. I can't find anywhere what the new default settings are, but turn them off anyway.
4 - Disable any programs you have running in the background while you play. Particularly things like P2P programs, and Anti-Virus programs. Don't worry, you can turn them back on later.
5 - If the worst comes to the worst, lower your resolution. I play on 1400x900 (Widescreen), so I accept that my FPS will be a bit lower, dropping it to a lower setting would, usually, improve the performance quite alot.
6 - Disable any addons that aren't essential. They might only be small, but every one of them is doing something in the background. You don't have to get rid of them, in fact, if you download another addon (like ACP - Addon Control Panel) you can set profiles for your addons, and load / unload them as needed. Do you really need Omen or BigWigs when you're solo'ing?
7 - Run a scan for Spyware (Use Spybot S&D or something similar) to make sure there's nothing using your network connection, that shouldn't be.
8 - Check what runs in the background when you turn your PC on. To do this you can either use a 3rd party program, like the free one built into Spybot, or go to the start menu, and type 'msconfig', select the 'StartUp' tab and it will show you a list of what loads. Disable any that you don't need. The advantage of using a 3rd party program to do this, is that they usually give you a bit more info on what they programs do. Don't disable any needed ones, and if you find any problems just re-enable them.
9 - If you're using a laptop make sure you're not blocking any of the vents (particularly the ones underneath). Overheating causes a lot of problems for laptops. The same applies to desktop PC's but it doesn't happen as much.
10 - Reboot - Yeah it really is that simple, sometimes just re-booting your PC solves everything.
Edit: If you're a MAC user, don't start spouting about how much better thay are, just sit there and look smug.