Personally I think the players coming up with some cool new trick for each encounter sounds pretty good. The problem is when they find one cool new trick that works for everything. Like, casting Create Water in someone's lungs sounds awesome the first time you do it, but you don't want a whole campaign of just that. But even if the players agree that that would be boring, it's hard not to do that without justifying why it wouldn't work, and if it wouldn't work every time, why would it have worked the first time?