I'm not saying Warlords of Draenor is the next expansion, but if "nu tablet pcs" is your only reasoning behind it not being the expansion that's about as hopeless as the people who were convinced Mists of Pandaria was going to be a mobile game of some sort.
The Warlords of Draenor trademark is aptly timed, and current speculative theories fit the lore (Draenor being the name of Outland prior to it being torn apart, Wrathion hinting heavily at an assault from the Legion, Draenor being in the past, the Bronze Dragonflight suddenly getting very involved with us via the Timeless Isle).
This segment is what people are using to say it isn't the next expansion:
I think it could easily just be Blizzard expanding their trademark to cover all the bases. World of Warcraft has a mobile component with the Mobile Armory. Additionally, within the last two years Windows 8 and developments in the computer industry have resulted in things like the Surface, a computer which can be a tablet PC or laptop via touch screen and detachable keyboard. If devices which are able to run World of Warcraft are expanding, it only makes sense that the trademark expands as well.Downloadable electronic game software for use on portable electronic devices such as mobile and cellular phones, laptops, handheld computers, and tablet PCs