There is currently no difference in the visual feedback between "Move" and "Summon", e.g. for Brother of the Wolf. This makes it uncertain whether I'm about to move the piece or summon. I'd suggest adding arrows to the "Move" indicator. Visual feedback on mouseover would also be nice, similar to the attached image (created in Inkscape).
“I will do this. Nothing in my life matters except this. No moment of my life exists except this moment. I am born in this moment, and if I fail, I will die in this moment.” Raistlin Majere, The Soulforge
Another (additional) way of doing it would be a different mouse cursor for moving, summoning and casting. That way you would see that you still have the spell or ability selected before clicking another unit.
Thanks for the nice suggestion, the game feels much more rich and easier to understand this way! Even though I disliked making it (I hate drawing tile graphics, it's so monotonous and somehow manages to take a whole bunch of time!)