A personality that engages with an audience on a level to the point that you are willing to pay money to see them lose. I heard so many people talking about how they wanted Manny Pacquiao to humble that cocky fucker Floyd Mayweather, and made special trips to the pub to see him get knocked the fuck out. That's a heel to me. Brock Lesnar is a great heel in UFC. He's the big fake, phony wrestler that shouldn't be able to hang with "real fighters."
I don't consider it a set of snarly qualities or pre-determined mannerisms. Just because someone is twirling a mustache, it doesn't make them a heel. Those things can be symptoms of being a heel, because there are generally a bunch of qualities in human beings that we have decided irritate us. But one of the things that seems the most bush league in wrestling to me is when people try and play a heel instead of simply being a heel. They're almost relishing in these preconceived notions of what heels do, and it becomes such a performance that they end up being graded on that. Part of the work, it would seem to me, is being able to calibrate yourself in front of whatever crowd is there, and do what you need to do in order to get that desired emotional response. Too much cart before the horse in being a heel these days.
Sometimes being a heel means kissing babies and being "so glad to be here."