You guys have all made perfectly good arguments and points about why blizzard does not allow account sharing or trading. I personally think a big contributing factor is that, there is already a huge market for gold selling, account buying and selling etc. Account buying and selling companies hack peoples accounts so they can sell them again. Thankfully there are probably more honest people in wow than dishonest.
If you think about how much of a demand there would be for account selling companies if it was allowed. Sure , I would love to buy that epic warrior or that awesome priest, but i don't since it is not allowed and I can see why not. If it was allowed, the amount of ingame spam, hacking and scamming would increase 10 times more.