Initial impression: the game has serious potential, but I don't think it's my cup of tea. My background is that of a Dota player, so I'm coming from that.
They are taking the traditional Blizzard approach here: good but not perfect game play, with an absolutely insane level of polish, streamlining, and quality of life changes.
They're very clearly going for "constant action" with this title. It's almost to the level of Super Smash Bros. Mana regenerates quickly, and creeps drop health orbs that heal you (similar to Diablo 3). The end result is, although you have a hearthstone that sends you back to base for healing like in LoL, you never really need to use it. All my games I've ran around outside of our base the entire time.
The talent system is a cool option. Every other level or so, you are offered a choice between two ways to modify your character. When you earn your ultimate (or "heroic ability") you actually choose from between two different ones as that level' choice. The end result is that you play every game a little different. It helps keep things fresh.
I'm not a fan of the f2p system they're going with here. I hate hate hate hate LoL's system, and that they are using it for Heroes is a massive turnoff for me. I'm currently limited to the weekly free hero rotation, and I can't try out a hero I thought looked really cool. This is opposed to Dota, where I can try out any and every hero as soon as I install the game. It feels limiting and like a cash grab.
Heroes have mounts, and I think it is terrible. If you want to, you can use your mount ability to give you hero a +20% movespeed bonus that is canceled upon dealing or receiving damage. Mounting up has a cast time, and no max duration. However, the difference it provides feels negligible, and watching Arthas or Diablo riding around on a ******* basic horse mount (as in, the level 40 human racial mount from WoW in all its basicness) is a horrible wrong that needs be righted.
The game is still very early on in development, obviously. The tutorial has Uther guiding you through, and it's really awkward to see characters speaking and their mouth not moving, since the animation isn't there yet. The female Demon Hunter is also ugly as sin. Certain UI elements have an obvious placeholder look to them.
All in all it seems a lot like hearthstone: a good way to spend a casual thirty minutes, not a super in depth competitive title.