I want one of two things from RPG combat.
If it's going the "action RPG" route then I want the same things I want from an action game. I want responsive controls, excellent animation, a feeling of weight and impact, a decent amount of variety in the actions available to me, and at least enough challenge to keep me on my toes. In short, I want it to be immediately and viscerally satisfying.
Games that have, to varying degrees, delivered on this for me include The Last of Us, The Arkham series, (and Shadow of Mordor which had very similar combat) Mass Effect 3 and the Mirror's Edge games. If you're thinking that almost none of those are RPGs then you're correct. That's because almost no action-RPGs have ever met my standards.
If the game is going the "tactical RPG" route then I want it to be turn based or at least allow me to issue/queue commands while paused. I want it to be well balanced, so I can't just find one strategy and use it forever. I want it to be both challenging and flexible enough that I can come up with creative solutions and feel clever. I want variety in the kinds of challenges presented.
Games that have, to varying degrees, delivered on this for me include Divinity Original Sin, The Banner Saga and the new XCOM games.
I don't want floaty, weightless action where I wave a sword through an enemy and their health bar goes down a bit (Dark Souls, The Witcher series). I don't want a bad shooter with RPG elements tacked on (Fallout 3+4, Borderlands, Deus Ex Human Revolution. I don't want anything that resembles Diablo. I don't want anything that resembles WoW. I don't want games where success or failure is largely determined by stats and levels (Borderlands again). I don't want games that think challenge involves giving enemies huge amounts of health (too many fucking examples to count). I don't want to be just mindlessly mashing the attack button (Skyrim).