His ability to withstand an insane rush without collapsing. I understand that it is hard to wrap ones' head around. And maybe It's the worst time for me to call him out as a QB of the future because of it, but I think that seeing a QB at his worst, while performing admirably, is a little bit more predictive than him performing at his best when he has an easy ride.
I know, that's not a great explanation. Maybe this is purely from emotion, but I have never seen a crappy Ravens QB perform like that in what is possibly the worst game of his career. It's not something that is inherently easy to explain. So take it as a grain of salt. But I've watch the Ravens since 1999 more or less every game; and I can't say I've ever felt that good about a sh*tty performance from Testaverde, Banks, Dilfer, Grbac, Redmond, Wright, Boller, or any other tragic QB we had until Flacco. And even with Flacco, when he had a terrible game, I am not sure I had hopeful feelings about his future. He was a different QB entirely when the playoffs were on the line. Not the same expectations I had then. He blew my mind with the 2012 Superbowl run, and I admit that.
The Bills O-line was crap against the Ravens. You fix that, and give Allen an extra second to throw, and he can be awesome. Don't be the Colts and ruin a good QB's career by failing to protect him (I know "you" have nothing at all to do with that). After Luck, I think any team can learn from that. I am sure the Bills can.
Also, please look at some of the better QB's in recent history. Their first season sucked. Their second season was not incredibly impressive. I don't think Allen is the next Peyton Manning or Pat Mahomes. I am not suggesting that by any means. But I think he might potentially be capable of being Elite if the offense continues to be built around him, and the defense is this good to give him the opportunities he has to make a big difference. If his defense suddenly sucks, I would not say that he can lead the Bills to a superbowl despite that. I have faith; Not a dream.