Really impressive considering the time constraint! You said this was a demo, does that mean you are planning a full release?
I understand we had limited time, so take these suggestions as things to potentially add in the future rather than a critique of what's lacking.
Various sound effects for the obvious reasons of having feedback for what's happening.
For visuals, I'd recommend having damage numbers show up whenever damage is dealt. Also, would be nice to either have healthbars visible at all times on each unit or show up when hit. If not that then have the HUD show the portrait and healthbar of the units engaged in battle. Anything that'll give me real time update on current health and damages being dealt, really.
Better indication of what powerups actually are doing.
Have the selection outline only highlight the top face of the block rather than all the edges that would be visible to the camera. It's really difficult to know exactly which block I'm selecting with the current outline (same goes for the movement and attack block outlines).
I love turn-based tactics so I'm interested to see how this game develops!