Only the tone of the voices... But that is rather significant. The change between Light and Medium is not always the same it seems, the same with Medium to Heavy. Also the tone change does make the voices sound different in pronouncation, which makes them sound like totally different persons speaking.

Let me say it, I'm happy. Militia Cavalry will most definately be changed to Medium, I hate their hysterical voices.
But plenty voices are good as they are right now, such as Cilician Pirates, though I might just give them a test run to see if the other voices are better.