Einstein actually explained, in a way, what exactly he meant, or what he would have liked to have said. For this we have a chap named Oswald Veblen to thank, who overheard the remark, was duly impressed, and about ten years later had the idea of making an inscription above a fireplace at Princeton, which set Einstein mulling over a different version. He jotted down:
"Die Natur verbirgt ihr Geheimnis durch die Erhabenheit ihres Wesens aber nicht durch List"
The revised form of the quote, noted on the back of a sheet of paper with some equations, was recently put up for auction, and fetched a price of over 40,000 dollars:http://www.christies.com/LotFinder/lot_detail...
In this form the "List" is taken out from "Raffiniert", if you see what I mean, and he chooses "Erhabenheit", contrasting it with "List". He appears to take "böshaft" rather as in "böse Junge", naughty. ("List" in this quote is often translated as slyness)
Hence I suppose the word "subtle" does cover what he later wanted to say. However, what came out on the spur of the moment was "Raffiniert", which as people here have brilliantly pointed out can mean really crafty as well as subtle or sophisticated.
Well, Einstein would have known the English version?