My 1/8" headphone output is not giving me full level in one ear
Short answer:
With your headphones plugged in, go to Preferences > Sound, and check the balance between your L and R channels.

Long answer:
I read that some people did not hear anything through their headphones and had solved the issue by unchecking the "mute" option under the sound section.  Mine was already unchecked.  The balance was fine too, but my headphones were not plugged in.  When I put them in and opened up Preferences > Sound and looked at the balance, it was pushed to the right.  Sliding it center fixed my problem instantly!