Sometimes a one-liner is exactly what you need to write, but in most cases I do appreciate a post that gets a little more thought into it. While I don't go nuts enforcing this (I sometimes write a few words myself), I've also tried to provide extensive replies, when applicable.
And I think the best way to enforce this (besides contacting the members personally and asking them to put a bit more thought into their replies) is to provide a good example.
My teams and the most valuable members, plus myself, have always written great quality posts and it inspired others to do the same. I know, even 20 years ago, when I had my first forum, we had peple almost writing "novels" in their replies and it was so appreciated.
Also, Nomad's idea, setting up a minimum word count has worked well. I'm sure even sure I have now the setting up, but it used to be in place on my previous communities.