[Based on playtest material delivered on May]
Light armor = (12-15) + Dex
Medium armor = (13-17) + 1/2 Dex
Heavy armor = (15-18)
What are the advantages of wearing Medium armor when trying to get the best AC?
- If I have no Dex bonus, I'd go for the best Heavy armor I could afford.
- If I have a +1 or +2 Dex bonus I'd either go for Heavy armor or Light (depending if I'm eager to sacrifice 5 ft. of movement for +2/+3 AC).
- If I have a +3 Dex bonus or higher I'd go for Light armor.
The gap on base AC between Light and Medium armor is too narrow, and the additional 1/2 Dex modifier provided by Light easly covers it.
Personally, one of the things I loved from 4th edition is the disappearance of the Medium armor category. Dividing between Light and Heavy only allowed for a wider gap in base AC. A gap that is difficult to overcome with the relevant ability only.