To touch on the debate above:
As a DM, you should design the world the way you want to. No other way. Its your time and your hobby, not your job, and the PCs are not your customers or clients.
It is true that uneeded details are, uneeded details. But you never know, I have had too many cases where things aimed at certain players or campaigns didn't really matter, and too many where I found myself wishing I had made some decisions, taken notes, and clarified things--for myself--earlier.
And, of course it goes without saying that players have the attention spans of monkeys. But then again, you might be surprised what gets them interested or excited.
But there is an easy solution: just do what you want. So if you are bored by doing pantheons or religions, take them from somewhere. But if you are finding that you are really getting into your worlds mythology, then spend time on it and detail to your hearts content. That is the whole point of do it yourself: you can do what you want.