Here's my opinion for all cards:
Thunderbird: The creature itself is not really interesting, but its ability is. I think it would be better if it was a spell.
Necromancer: It shouldn't attack, else it's a bit overpowered.
Golem: nice idea if only the enemy can freeze the water golem.
Tick: simple and interesting.
Crusader: very overpowered, you could just stack damage again and again.
Soulsteal: I think it's the best idea so far. It's very balanced and looks fun.
Quantum flash: nice, but for the un-upgraded version you lose 4 and only gain 3... Drawing a card doesn't really help.
Outbreak: looks like a weaker version of plague.
Crusade: nice but not so creative.
Terror sheild: that's so overpowered, it would render almost all attacking creatures helpless in a couple turns.
Harmonic pillar: kinda overpowered too. 2 times more quanta is huge.