Okay, here's the definitive reason of why I hate Things Change (besides the reasons I've stated before). Bringing back Terra, just for Beast Boy to move from her again wasn't necessary at all. Beast Boy had done an adequate enough job of getting over Terra when season 2 ended, she was mostly just a memory in the back of his mind at that point. Was it really necessary to retread old ground just so Beast Boy would have to get over what happened to her again?
Yes, I get the fact that they were trying to emphasize that Things Change(pun intended). But there was absolutely no reason to bring her back just to show Beast Boy had moved on from her(...again). Terra fans aside, viewers are given no reason to care about Terra because we don't know how she came back. It's kind of hard to care about a girl that Beast Boy interacts with when we don't know what happened with her. Thus my compassion for what happens to Beast Boy in this story feels hollow, because we are given no exposition in terms of Terra's story.
For the record, I thought Titans Together did a better of showing that Beast Boy had matured and had become a more competent hero than Things Change did.
In conclusion, I know that the writers tried to do a sad moving on story, but I just find myself not caring for the story because to me find the plot holes and unanswered questions to be road blocks for my enjoyment. But for the most part, it's because retreading old ground (like a Cyborg episode) wasn't necessary in the grand scheme of things.