Some of the data goes in the Registry, so if you want to call it a good system then you've got to include that too. You can't put the data in My Documents either since that'd be against the whole point of the Windows data folder structure you're defending. If you're putting it there then you may as well use C:/Blockland.
~faceplam~
Windows XP and newer versions have a Saved Games folder either in My Documents (XP) or directly on the user (Vista/7). That's where the game addons and saved builds would go.
Programs should never be installed to the root of the drive. Not only is it disorganized, it's tacky.
Programs should be easy to back up. When everything is stored in the user's folder, copying the entire folder can be considered a complete OS backup. All the programs can be reinstalled and then the user's folder is just pasted back in place. Blockland can't be backed up like that, while everything else I use, can be.
Also, the registry is rather abused at this point. Originally it was only used to store file extension associations. This was as early as Windows 3.1.