The loss of RWBY's charmingly snarky, sassy dialogue after the end of the Monty Oum era
The series had a lot of snippy, snarky humor during the Monty era that it later.... didn't. It didn't feel like character development so much as the series losing one of its main pieces of charm.
You can contrast with something like The Magicians, where the characters start out as sassy and abrasive to each other and later grow to love each other, without losing their edge. It might seem like a weird comparison since RWBY is an animated show about enthusiastic teenagers that has a bunch of combat scenes, and The Magicians is a live-action show focused on new adults jadedly problem-solving their way through fairy tale worlds with less whizz-bang action. But both start from enough of a similar Hogwarts-esque premise that I think this is a fair contrast to weigh RWBY against.