About the minimum payout: Something I would really like would be able to make auto-payout at a given frequency instead of a given amount. I've chosen my auto-payout amount such as I get payout every two weeks, but I keep adjusting the amount because of the constant increase of difficulty. So I'd be really happy if I could indicate I wish to be paid every two weeks, whatever my balance is at that point.
I don't think basing your minimum payout on difficulty is a good idea.
You want to have the same size coins coming in, as going out.
So if you spent $1 amounts all the time, it makes sense to have payouts around $1 worth of bitcoins.
On the other hand, if you buy more products at $500 value, you'd want a minimum payout close to that.
$20 value seems like a reasonable expected-spending price, hence the default.
Side note: Contrary to popular myth, difficulty increases do not influence price. Price does influence difficulty to a degree, but difficulty is so far behind price right now that it's not a good feedback loop.