My goals are to challenge the players via their PCs and create an enjoyable game.
What breaks spells for me is generally not power. I'm really not overly concerned with balance. 3e was a bit too unbalanced for me, but I still loved it. 1e and 2e were fine. What breaks spells for me is when they allow the group to bypass significant portions of the game.
Pre-5e teleport and Rope Trick are prime examples. The doesn't actually bother me, because it's visible and the world can interact with it. The Rope Trick being both invisible and placed high up in the air means that it will almost never be interacted with.
Maybe I've just been spoiled by playing with a bunch of highly intelligent and highly creative people for so long. It generally only takes a few minutes for them(or myself) to come up with something great. And I'm not saying that everything will work.