Collection of Thoughts from a 'Main Tank' player on 6v6 after about 2 dozen pick up games/scrims and a lot of quickplay with a Tank Duo.
The big thing initial thing i haven't seen people really talking about is the OW1 problem of Tank Duo's still being oppressively strong in higher ranks still exists. There's usually a plethora of DPS and supports in queue so odds of finding relatively close together ranks isn't a crazy ask and relative disparity in skill level can sometimes be overcome. But for Tank's smaller player pool and the fact that tank is actually 2 roles hidden in 1 the matchmaker really struggled to find games that weren't complete stomps when a Masters rank Tank duo queued. Either because it clearly couldn't find Tanks with MMR near us and just widened the search, or because the other team had terrible Tank comps like 2 off tanks both fucking off to play for off-angles.
As the playtest went on and tank popularity dropped things only got worse and worse with time.
Now just a collection of other ramblings and thoughts on meta, balance and the rest:
I think people have a more idealistic view because they were playing Quickplay which is often much wider MMR ranges and has people playing casually without optimized meta comps. But I think Competitive and Ladder ranked 6v6 will be stricter on team comp compared to 5v5. In 5v5 there's already tempo comps where a large chunk of the cast just can't physically stop a rush/dive (like a tracer+genji trying to stop a juno+mauga rush without ults). In PUGs/Scrims a Juno+Mauga+Dva or Zarya rush is CC immune covers stupid amounts of distance and Mauga get either bubble or Matrix to give him more damage or CC mitigation and you now have somewhere around 1000-1200 or hp/armor worth of tanks to deal with.
More in that spirit for solo queue and the frustrations of random teammates: In 5v5 low Hp/s support comps suck when you have a single tank splitting that between 2 in 6v6 is just as dog shit an experience as it was in late OW1. Having any kind of high sustained damage is so much more crucial in 6v6. Running like Widow+tracer+moira+Mercy takes fucking forever to kill 2 tanks and some combos like Rein+Sigma will pretty much always have barriers to throw out.
Random Thoughts on Tanks in OW1 vs the Playtest: Thanks to the removal of all the old support+DPS stun chains that could insta-gib tanks, Tank are in a weird spot of the best answers to tank aggression and coordination tends to be other tanks (Ana especially is stuck drawing circles in the dirt when you have an off-tank Dva eating up her abilities), this is probably not a great thing because tank vs other roles impact disparity was a large problem OW1.
The DPS passive does not really accomplish or solve what CC did in OW1. For a lot of tanks it tends to do very little when the tanks are actually burning their Cooldowns and engaging and more so punishes affects after Tanks have made their play. Mauga+Dva for example shares resistance and life steal for 3s when popping cardiac and then once cardiac ends Dva has DM for another 2s. A 15% healing reduction during all of that has an affect but it doesn't serve in the same way Sombra just shutting off all your abilities for 5s (on a 0.65s cast time with less LoS checks), brig having .75s hard stun, Cas a .8s stun, Mei primary being a DM piercing stun, and the CC fest that was DPS Doom when he was viable had.
TL:DR All that rambling it isn't to say i didn't enjoy the 6v6 playtest but by the end tank queues were already very short and most Changes you'd need to make to 6v6 for Ranked/Comp would probably be things that would make tank even less popular (like nerfs, doing something about tank duo's, and probably something about CC and bringing the other roles more line with how strong tanks are).