No matter what phase you enter in, you can still hang at classic 39 even if you don't have the top rated BiS gear. There is enough room for skill to allow you to play without GF'd gear, biss rolls or naxx shoulders etc. Of course the hill is steeper to climb, but you can certainly climb it and be a top tier player even if you aren't wearing the most favored BiS gear.
If you want to play a certain class, just be sure to do your research and make sure you are setting yourself up for success.