My complaint is that the flash sales on GOG end at weird times. I saw something on sale the other day, it looked interesting from the "box" art, but I'd never heard of it so I watched the trailer for it and was convinced that I should buy it. When I added it to my cart, the sale had ended. This was at something like 20-25 minutes after the hour. I would have expected a sale to end on the hour, or at least in half-hour intervals. Not at 6:22 (for example).
That really annoyed me.