Play-from-tray as a playstyle means focusing almost entirely upon powers, specializing on doing most of your damage with powers while minimizing any weapon use. The weapon playstyle, naturally, is the inverse; almost all of your points go into maximizing weapon damage and your powers are secondary at best. The goal is to ensure that damage-oriented players can be dealing just as much damage using a powers-only approach, a weapons-only approach, or the usual hybrid build. Check out the latest dispatch for more details on how everything is being balanced.
Source: Official Site