I get the feeling almost everybody uses an editor (Vim, Notepad++, etc.) for the Git commit messages. Why?
I find typing -m and a couple of quotes is easy and provides a easy way to redo a commit (pressing up arrow). I guess it’s easier to do multi-line commit messages in an editor, but I have a hard enough time convincing fellow co-workers to write any message!
You mentioned the most important thing – length. Commit messages should essentially always be multi-line. The only exceptions are trivial commits (e.g. “bumped version number to X.X.X”) or merges with no conflicts (though even then, appending a shortlog isn’t a bad idea). The average commit, as far as such a thing exists, should probably have a sentence or two beyond the subject; some could even have paragraphs. Just look at the log of git.git; it’s pretty much guaranteed to be a good example of commit message style and length.
I realize that it may be hard to convince others to write good commit messages, but that doesn’t mean you can’t – and you will probably find it easier to use an editor to write them.
(I have exactly the same experience in my workplace. My coworkers are engineers first, programmers second, and version control users… third would be generous. But you can at least do your job right!)