Prado Dam is the biggie. Giant project on that right now that still has yet to wrap up. The other hitch is Green River Golf Club. The Santa Ana River runs right through the course, which has been privately owned for many years (though Orange County has expressed an intent to buy it, which would solve some problems with the trail project). So you've got the legal hitch of cyclists riding through private property.
Years ago, mountain bikers could connect with the Aliso Canyon Road in Chino Hills State Park through Green River. Now you can't do that. If you drive to Green River and take a look to see whether you can still cut into Chino Hills State Park from that entrance, you will a) notice cameras trained on you, b) see a security-cop golf cart quickly heading your way. GRGC does not want to see any trespassers on the course.
You've got GRGC, the Army Corps of Engineers (Prado Dam) and possibly the railroad and METROLINK to deal with in terms of ironing out the right of way. Very complex.