The thing that bothers me most is when the singer is purposely disrespectful to the anthem. Of course, this is exceedingly rare; the only such occurrence that comes to mind is Rosanne Barr back in 1990:
Barring outright disrespect, I think what bothers me the most is when a singer tries to stylize it so much that it effectively becomes a different song. R. Kelly came close to that:
There are also a couple of nitpicky things that annoy little ol' anal-retentive me. First is the word "perilous." For some reason, it is rare that singers pronounce that correctly. they ususally sing "pero-lous" pronouncing the first two syllables like the name of former presidential candidate Ross Perot, or "perilless" which means "without peril." The other nitpicky annoyance is when scoreboards flash the words so the crowd can sing along, but get the punctuation wrong. There are questions in the anthem, and rarely do I see a question mark.
At any rate, let's end things with a montage: