Alaska and Hawaii are obvious. Anyone else able to spot the other 7 missing states without singing "Fifty Nifty United States"? I confess I almost got through the whole list of states before figuring out the last missing state.
The Dakotas, Nebraska, and Kansas were the first ones I noticed were missing. Delaware, New Mexico, and Pennsylvania were the ones I needed to song for.
I had to check ExplainXkcd to get the joke at all; I didn't notice any of the missing states at first.