So as it turns out, it's not that we couldn't favorite videos anymore, it's that the "Add to" icon has been replaced with an icon with three horizontal lines and a + . . . a bit confusing.
But hey, I'd be cool with YouTube conforming videos to their aspect ratios and removing the black bars. I actually wish YouTube had been doing this all along. I mean, 16:9 videos fill the entire video player (if they're exported correctly), I don't see why the player can conform to 4:3 videos without pillarboxing it.