Unable to start any application
It looks like a coincidence to me. 

Only the files and folder in ~/Movies should be affected.

Even running the command with sudo in the root of my drive doesn't render my system unbootable:

for f in .*; do sudo mv "$f" "_$f"; done


Some files and folders are renamed (e.g. _.fseventsd, _.Spotlight-V100, _.Trashes, _.DocumentRevisions-V100, _.vol) but after a restart the important ones will be recreated with the proper name (leaving the renamed folders as they were).

Navigate with Terminal to / and /System/Library/CoreServices and check if anything was renamed; if not reboot otherwise remove all the underscores.