Does your mix sound distant?
Try using less reverb and effects.
Can your hear every lyric?
Every word must be heard. That's what automation is for.
Can your hear every note being played?
Automate to hear every note.
Are the sounds dull or uninteresting? Are you using generic synth patches or predictable guitar or keyboard sounds?
Does the song groove? Does it feel as good as your favorite song? Is the instrument that supplies the groove loud enough?
What’s the direction of the song? Should it be close and intimate or big and loud?
Are you compressing too much? Does the mix feel squashed? Is it fatiguing to listen to? Is all the life gone?
Are you EQing too much? Is the mix too bright or too big?
Are your fades too tight? Does the beginning or ending of the song sound clipped?
Did you do alternate mixes? Did you create at least in instrumental-only mix or TV mix? Some clients need them, others don't. Ask first.
Did you document the keeper mixes? Are all files properly named and marked? Are you sure which file is the master? Is it color-coded?