There was a vocal minority of players who were pretty upset when the old MSAA options were removed in patch 6.0 in favor of visually inferior (in most cases) post-processing AA options. Heck, Blizzard even wrote a technical blog explaining why they decided to remove MSAA. In short, their reasoning behind its removal was because it caused too big of a performance hit.
This wasn’t surprising since MSAA is very performance unfriendly, especially if you run the game at a higher resolution. Blizzard has always aimed for WoW to work on the lowest common denominator of PC hardware, but many argued there was no reason to take away the other AA options from people who do have the hardware to utilize it. I personally have a GTX 780 video card which is overkill by about 5x for WoW, but it is nice to be able to turn on every single option to max and still run the game at 60+ fps at all times.
The good news is that Blizzard listened and they have re-implemented MSAA as an option in patch 6.1. They’ve also added two new options, both even more performance hungry than MSAA.
Update: Since this article was written, Blizzard has added support for post-process AA (FXAA or CMAA) while running MSAA, under advanced graphical options.