I think it's a combination of tanks losing some cross class skills they were used to, changes to skills they were accustomed to, and changes to their stats. Tanks are trying to re-learn, in a way, how to mitigate damage, but they're still pulling like nothing's changed, when it has. So far I've been able to keep up as healer, but roulette has gone from a pleasant experience to anxious. If I'm running with friends it's no problem, but suggesting to a tank you don't know that it might be best to slow down tends to end in a lot of "Don't tell me how to play my job" conversations.