Mearls has expanded a little on this on
his Twitter.
It looks like all classes will progress from +1 to +6 at the same rate.
If you aren't proficient in something, you don't get the bonus. If you're somehow specialized, you get double the bonus.
I have mixed feelings about this. It's simple, which is good, and should allow the skill system to be optional on a player by player basis. But it also lacks nuance.
Also, because no class is going to get double the bonus to attacks, all classes get the same attack bonus. I can live with that, but it feels a little strange.