If blizzard stays on it's trip of totally outdating almost all relevant content when a new expansion hits, they really need to roll back all old expansions into being free. You essentially pay for cata to level for some hours now.
If I had it my way, I'd just make it so buying the current expansion gives you all the past expansions and base game, and a free month if you're new. That'd make more people more interested in the game, I feel.
I'd say dealing with the game itself, the wonky and outdated game design that doesn't teach you anything, really, for endgame play (the focus of the game), and the horrible community are the big things holding this game back towards new people.