I find that I get more done overall when I only put due dates on things that were truly do-or-die deadlines. Things I truly need to do get assigned an appropriate deadline, but things I only want to do will get flagged. I grab from the latter list as I'm able once the stuff on the first list is dealt with...
Any chance we might get OmniOutliner-style arbitrary custom columns in some future version of OmniFocus? That way we could add our own columns for a "soft deadline" or any other metadata the individual user finds useful. (Contact links come to mind. Sure, we can just dump everything in the notes field already, but more structured containers are often easier to work with.)