I made this post when the Browns didn't trade Njoku and Jets didn't trade Hall at the deadline. And unless I'm wrong the Browns got no compensation for him signing with the Chargers (Hall re-signed so makes him sort of moot). Njoku really drives home the point here.
The salary cap and other factors make the NFL trade deadline the hardest to do deals among pro sports, but a big reason also seems that GMs just don't seem to have a mentality of getting anything for expiring assets like other sports.