I think a lot of it had to do with sitting down.
When he's at a podium he had a tendency to lean over on it. That made his suit coat ride up on his shoulders, making it look like he was a tiny old hermit hiding in a cave.
He stopped doing that the last couple debates - someone must have noticed and corrected it.
When he was sitting last night, he was chillin' out, relaxed, leaning back on his chair. The rest of them were all sitting straight up at attention (or, in Santorum's case, unable to relax because of the Rockwell 60 hardness D2 steel rod up his butt).
Plus there was that AMAZING split screen when Newt was talking and they put Ron up on the left, and he had his chin on his folded knuckles, looking at the floor, obviously processing what was going on in thought so deep that there are unknown species of lanternfish hanging out down there....
Whoever is working on his debating is doing a spectacular job. It's really showing. They're correcting mistakes, but he's still coming across as altogether different from the others.