Easiest way to extract the contents of many folders at once?

If you want to move the contents of a source directory and all of its sub-directories to a single destination directory (i.e. flatten the folder structure), it's very easy and you don't even need to bother with the command line.

Just go to the top-level source folder (whose contents you want to copy), and in the Windows Explorer search box type * (just a star or asterisk). This will display every file and sub-folder under the source folder. Now just sort by Type to group all files together, select them all, then cut/copy and paste in the destination dir.

If you have hidden and system files as well, under Organize → Folder and search options → View tab make sure Show hidden files... is selected, and Hide protected operating system files is unchecked.

You can use Windows Command Prompt for running a command:

move C:\path\to\folder\*\* C:\path\to\folder\

I am not 100% certain about dashes, but \ or / should definetely work. The above command will move all files from all folders in C:\path\to\folder to C:\path\to\folder.