The one thing that keeps bothering me with MAS updates is that you cannot see the download/install progress. It will show it at first, but then the progress bar indicator simply disappears after a short time. I have to close the App Store window and reopen it, often multiple times, in order to be able to see the download progress again. This is not an issue for smaller updates, but larger Xcode updates are a pain trying to figure out how much data has been downloaded so far.