An instant classic. I am quite pleased with almost every aspect of this puzzle.
Let's start with the theme. A nice 13-letter across the middle reveal, FALLINGBEHIND, which Cece suggested off of the clue and a few letters in the center. Even once we had that, and had entered GORYDET/AIL at 56A, I didn't see the actual detail I was supposed to notice. Not until all of the puzzle had been entered, did I see the behinds that were falling: BUM, RUMP, CAN, and TAIL. And such a wonderful thing too, for those of us who enjoy bringing up the rear, as it were.
Then there is the excellent cluing. It started in the NW with 12A: What's brewing? (TEABAG), but even better with 3D: "Hurray" or "alas" (IAMB). That's great stuff there. How about 23A: Like faces after face-lefts (TAUT)? Or 48A: Accords, e.g. (HONDAS) - nice hidden capital. I love 9D: "That'll _____ ya" (LEARN). We'll round it out with 57D: Figure that's unbelievable? (YETI).
Beyond all of that, there are the fine answers. GALACTIC is great, GODPARENTS is a very nice long answer. HEARINGAID fits both clue and answer (28D: Device placed next to a drum). WERELOST is awesome. PTERO was unexpected.
And there's James DOOHAN. Even though Simon Pegg's doing a great job of playing Scottie in the rebooted Star Trek, nothing beats the original.