1. The game has 9 unique characters which is great because they all play different just like Diablo 3 (except D3 has 6 characters).
2. The customization in Hearthstone is endless. You can be a Druid for example and have plenty of different builds. One can be a defensive build, the other a magic damage build, the other a minion buffing build, ect... The possibilities are endless. This element is missing in Diablo 3 as most people pick one popular build that overshadows the others.
3. Items in hearthstone are cards. You have rare, epic, legendary, ect... You get cards through crafting, buying (gold or real money), and doing arena challenges. Every single card is original, well thought out, and mostly balanced. Diablo 3 items has had a bad rep. If only every item was well thought out like Hearthstone.
4. Hearthstone is all PVP action and its a solid PVP experience. Magic is more like chess while Hearthstone is more like an arena spar (roman arenas). At first I thought the cards were too simple but the more I play, I realize how complex the game really is. Diablo is mostly PVE and will hopefully have a complex PVP system in ithe future.
5. Crafting is easy, fun, and very rewarding in Hearthstone but not in Diablo 3.
6. In Hearthstone you can rank up. You start from the 20th rank and work your way up. Trust me. This game is extremely challenging. In 2 weeks, I barely made it to the 16th rank but in the first week, I was bouncing back and forth from 20 to 19th rank. I can't imagine how well the players in the top 5 ranks play. It was feel marvelous to be in the top 5. I would feel like a bad a-s. There should be some sort of ranking system like this in Diablo 3 (imo ladder). The scariest part of Hearthstone is that you can lose rank by losing games. It really makes you worry about dying. I'm never worried to die in Diablo 3 and I have to ladder to climb.
7. And finally, if you get bored, you can play in an Arena where you randomly select cards to use. You won't have a good deck so its best to get plenty of experience out of the arena so you can go win. You really want to win in the arena because you get rewards such as arcade dust or packs of cards. There is no side game in Diablo 3.
Anyway. I think Hearthstone gets a solid 5 stars in my book. Its truly an epic game. I'm completely addicted to it. Its marvelous and a refreshing revisit to the experience of playing an epic game. This is what I expected to get out of Diablo 3 but it never happened. I hope that I will one day feel like I am playing an epic game when I play Diablo 3 but there have been too many disappointments. At least Hearthstone is great.