Alex Klontzas

For sure. I think that much like we did, the dev team underestimated what was needed to make a main tank "work". And more than that, they saw how some tanks seemed to double well as offlaners (like Leoric) and tried to reproduce that dynamic....with no success.

Is Leoric played as a tank rather than just offlane?

As a main tank, no. But he'd show up as a secondary sometimes while they'd put another hero on the offlane. The big difference between the two is that Yrel/Blaze are waaaaay more abundant and not as easily countered.

Not having to hit heroes to sustain feels like the big difference

Well, in part. There's some (like Thrall) who can heal off minions too, the difference of course being that Thrall can't eat nearly as much punishment. It's a combination of things, really.

Even Thrall can kinda be zoned from sustain, but yeah high durability + escape + free sustain. One option could be adding a mechanic where both heroes have a blue bar called "Mana" that allows long term pressure of their sustain

I dunno, that sounds pretty controversial. How am I supposed to spam my abilities if there's something limiting how many times I can use them?

I think any hero that doesn't have to hit enemy heroes for health sustain needs to have serious Mana considerations