iserith
Magic Wordsmith
I understand how liberal D&D is in it's structure. It's one of the things I like best about it. I'v thought about doing that discussion but just a session ago I limited an ability of his and he ended up requesting a character revamp claiming I basically destroyed his reason for having that ability (it was an invocation).
(here's that situation shortly: he had repelling blast. He hit a huge creature (as big as a five story building basically) and wanted it to move 10 feet back. I said he can't move something weighing hundreds of tons 10 feet back with a simple spell like that. He said because the invocation doesn't specify how big the creature must be then it should work on a creature of any size. We argued over it for half an hour. In the end he agreed with me reluctantly but requested to unchoose repelling blast).
As an aside to the aside, my recommendation as a longtime DM is to not find yourself in these arguments by just siding with the player. You're on the same team after all and moving a huge creature 10 feet probably isn't a big advantage anyway. (At least not all the time.)