I just noticed a nice little feature.
When you give an inactive friend (who already had a WoW account) a TBC trial and that friend pays again for at least 1 month, you get a free month. So basically 2 can play for the price of 1.
I quit, but my brother still plays.
So when my brother almost needs to pay for another month, he'll give me a TBC trial and after that I'll pay for another month of vanilla WoW. He'll get a free month. We'll just share the costs.
I'll spend my month searching for pets and mounts.
And boosting my lvl 60 dps.