Poking around the checkouts I see that you buy the game individually for each operating system as opposed to buy once, run anywhere. If my main mac dies (again) I'll have to shell out again to play it on a Windows or Linux machine. Not a huge fan of this sort of arrangement, I liked Braid where I've paid for it once and have access to Windows and OSX copies.
Actually, there's an opportunity to purchase "insurance" or the like for an additional $5. The deal is that you have access to the source and can download as many times as you like from it for a one year period. So three games for $10 and a one year warranty, essentially. Not a bad deal.