A clever but simple thing I've noticed that STS does to add some tactical crunch to combat

is that a lot of enemy attacks do amounts of damage that are slightly over a multiple of five

6, 11, etc

and the reason this is a substantial change is that the default defense card every deck gets, Block, blocks 5 damage

so if you have an incoming attack for 6 damage

you can play one Block, and just eat the 1 damage

or you can play two and not

but playing two cards when you have 3 energy is a serious investment

so the constant question is 'is it worth avoiding one or two points of damage'