(Flash Thoughts are very brief statements.)
There's a lot to praise about Slay the Spire, but I'm particularly enamored with how much the UI adds to the game.
(Barricade achievement secured)
They made playing cards feel as good as possible. Note all these little tricks they do to make the game feel smooth and feel good.
Cards have little streaks when you draw/discard them. It's pretty and also helps you understand the flow of the game.
They perfectly lined up the "cast zone" to make flicking out targetless spells super easy and quick.
You can cast your cards as quickly as you'd like, and the animations will just do their own pace -- you don't have to wait for an animation to finish before doing the next thing. Most digital card games do this, but it's still important to take note of.
On a similar vein, you can cast cards before you're done drawing your hand. This is really nice for when that louse has 2 HP and you just need one strike.
The "hitboxes" for targeted attacks and spells are absolutely massive, and enemies are very spaced out, so you will never accidentally target the wrong person.
The changing icons of the weapons at new thresholds of damage is a great way to help players intuit the severity of the situation, in a way that feels more visceral than just numbers.
Satisfying, "exaggerated" spell effects and sounds make the cards feel like they really have weight and physicality.
The sound/visuals of ethereal cards like Daze when they disappear are #1.
Flash Conclusion: "nice"
Maybe I'll talk about design later, but also maybe not.