Very fun game, I look forward to its development!
Its still early in its production and there a number of flaws, many of which I have seen mentioned in comments, but one I have not:
Druid is seriously overpowered, especially with the second Rune and when combo-ed with the Mirror, something easily doable later on with the Sustain transient rune (and Amass buffs it even further). I have yet to find a strategy that would even remotely compare. Also the double card improvement equipment makes it even crazier.
Oh and another one, Forge and other yield upgrading cards swiftly make starting card light yield almost entirely irrelevant and not worth upgrading, something not helped by the doubling from that one equipment.