You get this message because you don't have the right wow client verision(duh).
You probably just upgraded your version to 2.4.3. We currently only support 2.4.2.
You can downgrade by running the repair.exe located in your wow folder and then redownloading the correct patches.
If I was you, I'd just wait until we had 2.4.3, which shouldn't be too long from now.
We level faster than retail, if thats what you mean. 25x faster(or 26 or something, I forget).
If you're confused about downgrading, I'm sure someone else will see your post and provide with a link to a previous post about this same issue.
In a related note, I love repeat posts.
