If you are looking for new content, then changing to Alliance won't offer much more.

The first 10 levels are specific to the race, the next 20 I suppose are more specific to the "allegiance" [and still differ depending on where you decide to level up]. After that, the same areas are opened up to both.

However, there is a ton of stuff to do. For example, I have been to Blackfathom Deeps and Dire Maul once. I don't even think I have ever stepped inside Uldaman or Maraudon. So, the content is probably there if you really want to experience it.