Someone in one of the other threads mentioned that UA was testing some new methods with DIA as the guinea pig. Lucky us.

If this "check your carry-on, get on early" becomes a repeated practice, I think it will backfire, that people who could have and would have checked smaller luggage when they check in will now take it to the gate with them, hoping for this opportunity. If the opportunity doesn't emerge, there will be more (maybe significantly more) bags being carried on, making the situation worse than it was before the practice was started.

IMHO, much of the carry-on problem is due to the fact that UA (and other airlines) don't enforce 1 carry-on plus 1 personal item rule. Stop people from boarding with 2 suitcase sized bags and require them to check one a few times, and they and others will start to get the message.