One major reason people have trouble finding quests in Shadowmoon Valley especially at higher levels it they miss picking a faction in Shattrath, you need to have picked Aldor or Scryer to get access to a whole bunch of quests in that zone, Netherstorm too.
This is the starting point to pick your side, and the faction you pick will then usually point you in the right direction.
Something else you may find handy is the Completed Quests Checker on IcyVeins, been a big help for me to find some stuff I’ve missed: