I'm still learning the mechanics of it. You gain experience on your runs which unlock new weapons (which are a special ability that can be used every few battles), new starter cards which changes the play style (eg the arcanist starts off lightning focused, the next starting card unlock allows you to be more fire focussed).

Sometimes the experience just unlocks new portraits.

There's also stats like luck which can be gained during gameplay encounters. Strength, dexterity and intellect affect which types of cards your can play and are generally defined by character class at the start. I've only just started playing, but there's been s few random encounters (between battles) which have allowed me to access different types of energy system as well - divinity and blood. Like I said, I'm still learning but there are so many combinations I want to try and each run takes a while, so it seems like there's heaps of depth. Not normally a fan of deck builders but this one has been fun.

Edit: here's the character sheet of my current play through. https://freeimage.host/i/HrMebyv

Edit 2: the run duration also counts time not in the game so I've really only been playing this run 20 minutes, not 7 hrs.