Having the start menu be an overlay instead of full screen is much more functional. Your focus can remain on the windows you have open. With the default full screen start menu, your focus is forced 100% onto the start menu, which is generally unnecessary and frequently disruptive. One example is if you are working on spreadsheets in excel and you need to open open another program real quick (this happened to me the other day when I had to open a program to run statistics on my data). The full screen start menu yanks you out of excel and fills the screen with a grid of tiles. After I'd selected the program and was back in desktop mode, I was a bit disoriented when I went to my spreadsheet and it took a few seconds to figure out where I had left off. With the old menu, my spreadsheet was constantly in my peripheral vision so I never lost focus. It may seem nitpicky, by when you spend hours in a row crunching numbers thers little interruptions can be extremely annoying. This also applies if you are watching videos, editing photos, etc. Just no need for a keyboard and mouse user to have a simple menu become a full screen grid.
Not to mention the default windows 8 start menu doesn't have folders as far as I know. I don't care about grouping my programs together, I want them hidden in folders. Instead of a 'games' category that takes up half my screen, why isn't there a simple option to put them all in a single folder in the start menu?