A: your ideal spot for a necro. He can control all of the area outlined in red, which includes the gem tile! You can protect this necro by having a bare VM pop from the bottom deploy tile and move forward 4 tiles (at the E). Very AP-efficient!
B: beware of archers here. They can hide behind the crystal, dash out for a quick 3 hits and hide behind the crystal again.
C: a priestess here can get an extra 20% physical defense (generally not very needed) while being able to rez/heal a unit that's on the gem tile. A good central location that should be able to heal most of your units. (including the VMs on E)
D: an imp on D can use 1 ap to move forward, and killstomp a unit on the opponents power tile. Generally this will be a high value target like a sworded necro or archer. You can protect this imp by having a bare VM pop from the bottom deploy tile and move forward 4 tiles (at the E).
E: if you have any units standing here, a grenadier can stand on the gem tile and splash your crystal. This is less effective since the patch, but you should be aware nevertheless.