This is a lovely game that managed to enthrall me for multiple hours. It's very fun to experiment with different character combos and upgrades and see what works best for each dungeon.
I've also apparently found some memory leak-ish behaviour, over the time of a dungeon the RAM usage drastically increases (Firefox on Linux). Only at the end of a dungeon it's cleared.
As far as gameplay is concerned, there's really only 2 issues:
1) Mind control is too strong. 3 Witch Doctors with +1 speed and mind control upgrades are basically invulnerable. Meanwhile the most damage I ever took in a turn was when a Mind Flayer mind controlled my Mercenary from a mile away with no counterplay.
2) Most main attacks are too weak. The delay hurts too bad until only 1 or 2 enemies are left. Even the "strongest" ones like an upgraded Wizard's or Priest's are actually just as likely to hurt your own units as the enemy due to all the control available. It's much more efficient to use other moves to make enemies hit one another or drop into traps. So most of them need a little more oomph or utility at a baseline or on an upgrade to grant more incentives to use them.
I like the difficulty scaling. It's fun to get more upgrades in a harder game (and honestly, I'd love to see even longer dungeons).
Some of my favourite characters were the Barbarian with its great crowd control, an attack without delay and the ability to skip Recovery; the Priest with the potentially massive lightning AOE and an amazing support ability; and the Mercenary with the only main attack that doesn't have a delay.