From a Hero Design perspective, this is something we've talked about a lot on the team. In years past the conventional wisdom was that a "tank" had to be hard to kill, and do things that are sufficiently disruptive (damage/CC) that you can't simply ignore them - which in turn allows them to soak damage for their team.
However, around the time frame between D.Va and Garrosh, we sought to establish a more clear definition of what's required to "tank" for your team and what tools a Warrior needs to do that effectively.
This isn't a comprehensive list, but I believe we landed on something like the following:
- Durability/Mitigation (so they're not easy to blow up)
- Self-sustain (so they can stay in a fight)
- Peel (so they can protect vulnerable allies)
- Interrupt (so they can disrupt cast/channeled abilities)
We've found that Warriors who don't have enough of one or more of these qualities end up being used as bruisers to harass the enemy backline. Most of the time it's "peel" that's lacking, as this is the aspect that wasn't as clearly defined in the past. Even now I'd say it's hard to say precisely where that line is, and that it's subjective based on what other heroes are capable of.
If you look at someone like Sonya with 2 stuns, you'd think she could be a tank - and indeed to some extent she can, but other aspects of her kit push her to be more aggressive and limit how effective she can be at protecting a squishy backline (compared to someone like Muradin or Arthas). I think this is something we're trying to be much more aware of earlier on in the development and playtesting process for new Warriors. Do we want them to be able to tank? If so, do they have all the necessary tools? Are there aspects of their kit that make it unintentionally more difficult for them to do so?
This is something I'd encourage all of you to think about (and feel free to disagree with us on!) when discussing whether a Hero can tank for their team, and what they need to be doing to tank effectively.