That all depends. It could be hidden files for sharing. It could be a hidden folder for themes and such. It could be a number of things. Applications dont always install EVERYTHING to one directory. There are a few hidden directories that have important information for certain application.
So either you have to find all the files and folder for the game. Use the installer, if you didnt delete it. Or else you have to redownload it again. Nothing is for sure when you try to copy over Registry keys. Doesnt always mean that will work to make an app run. The installer is the only sure fire way.