AtomicParsley is a lightweight command line program that can read and set iTunes-style metadata tags in MPEG-4 files & 3gp assets in 3GPP/3GPP2 files.
Very useful app, but not actively supported. Several longstanding bugs. Fails about 25% of the time due to bugs. Unfortunately, this is the only Linux command-line tool to manipulate mp4 tags.
The command line client is useful for a small collection, but unwieldy for a large one.