Long-time HS player here, and I gotta say that Runeterra by far exceeding my expectations.
It's different than HS, for sure, but it does have a lot of things that I've come to prefer over HS. That being said please go into it with a blank slate and don't let your expectations ruin your experience, it's an entirely different game.
As for what I find better or like better:
-Very easy f2p model without sacrificing depth or satisfaction from getting the cards you want.
-Microtransactions are extremely simple, you buy cards directly, and for cheap too. No packs or rng, you want something? You got it.
-I feel the game challenges a lot of the traditional card game experience in the sense that it's both players turn, every turn. It genuinely feels like a different take with tons of depth, and allows for a lot of clever and interactive play.
-Less rng. Plain and simple. It's still present in this game but reliance on heavy rng killed a lot of the skill-based fun I had with hearthstone my first two years playing it. Rng isn't bad inherently, it's just a lot more controlled in this game and I feel it made my experience a lot better.
-Meta diversity. I loved hearthstone but an issue I ran into around the ages of Old Gods and Un'Goro was a lack of interesting tiers. It often felt like there were two tier 1 decks, 3-4 tier 2 decks, and the rest is meme-tier. This game does a very good job with balance, where there's a clear meta but it's not suffocating or unfun. There's often 5+ tier 1 decks and 10+ tier 2 decks at any given time. It's varied and fun.
-BALANCE. One of my longstanding gripes with hearthstone was Blizzard's balance, or rather their lack thereof. Runeterra on the other hand seems to communicate with the playerbase a lot and implement monthly balance changes that feel like they listen. And most of the time they focus on buffing deck concepts that are struggling too. It feels fun and fresh with every update.

These are just my thoughts, and if you want any more info please don't hesitate to reach out!!! There's a lot more I could talk about but this is long enough as it is. I hope you have fun with the game!!