Emotions. Opinions. Five senses. Or in a single word: depth.
Author Dean Wesley Smith has an extensive lecture series on what it means to have depth in your writing. It is well worth the money for the 7-course bundle, or you can go through each one individually too.
If anything, take the introductory Depth in Writing class.
Trust me, it's a game changer for sure.
Because after I took the class, my stories changed and were noticed more than before.