I have a ton of thoughts about Gloomhaven! But for now, here's just one: Gloomhaven's ability system is a veritable fountain of fresh design space.
The combat system of Gloomhaven is something like this: you have a hand of 10-ish cards, and every round you choose two cards to play from your hand. When it's time to do your turn, you play either the top OR bottom end of your first card, and then the OPPOSITE end on your second card.
In Gloomhaven, it's generally sorted out so that offensive abilities are on the top, while movement and utility abilities are on the bottom.
In this example you're down to 3 cards in your hand. Listed beneath the cards are a few potential combinations of stuff you could do for the turn. Note that you can't choose the top and bottom of the same card, and you can't have two bottoms/tops together.
This leads to fantastic depth. In particular, the fact that you mix and match different abilities (and eventually are forced to get creative as your hand size dwindles) leads to some really cool decision-making as you discover different synergies between abilities.
For the player: it lets you feel creative when you come up with interesting combinations of moves for the turn.
For the designer: it lets you give characters abilities that would otherwise be too OP together, because you can just stick them on opposite ends of the same card. It also turns otherwise ho-hum (design-wise) abilities into interesting key pieces once they can be combined with other abilities.