Current level -2 has always been the needed amount. You could enter BC at 58, but most struggled at -2. Generally the new beefier gear was worth the extra pain and after you replaced most of your junk gear with the new stuff it wasn't so bad. The first of those -2 levels was hard, second was easy.
At least that's how it was in the past.
You had to be max level to do Cata and MoP. It was a quest that granted you access to the new expansion.