I used to neglect blush...would you believe I've been obsessed with makeup for ten years and only bought my first blush this year?! I don't know if I was just scared of it or what. If I am short on time, blush is definitely the first thing I ignore.
Also lips...despite all the lippies I own, I usually just put something on when I'm leaving home, and then never reapply out of laziness or because my lips get all funky.
And finally (I neglect a lot, hehe), my poor brows. I had two horrible waxing experiences in the last year, so have taken to tweezing them myself which I seem to never get around to most of the time, and I have a hard time shaping them. They look best when filled in, but often I don't have the patience to fight with them.