A feature in 1.8 is supposed to be when you hold control and then middle click, it will store the NBT data when you place the block once again. For example, put items in a chest, hold CTRL + Middle Click on the chest, place the chest; it has the same contents.

This works for every block, except for command blocks. When taking NBT data from a command block, it does not store the command inside. If you were to put "/give @p stone" in a command block (make sure to hit done in the command block), use CTRL + Middle Click, and then place the command block once again with the stored NBT; there's no longer the "/give @p stone" command in the command block.

Not sure why it only affects command blocks, but it does in the latest version