Having performed the song several times before semi-major sporting events, I cannot fathom how people screw up the words unless they're a nervous kid or just blank on the entire song, which sucks but does happen. The words are really not that hard. Learn them right.
The warbling is oftentimes to disguise a lack of range or the inability to sustain a note without going flat/sharp. That song is damn hard to sing and stay in tune. It's got a fairly big range at an octave and a half with sustained highest and lowest notes, and people have a bad tendency to not think through the whole song when they pick a starting note. And honestly, I don't mind a little embellishment as long as it fits within the already written phrasing and rhythm. A trill here or there won't kill me.
The few times I did the anthem at larger stadiums, I was told I had a time limit (usually in the range of 90-120 seconds). Of course, this doesn't apply to your superstar performers, who are there as much to promote themselves as to sing the song.
My pet peeves -
Sing the damn thing in 3/4. The song is in 3/4. It is not in 4/4, no matter how much you like the Whitney Houston version. That was an arrangement that changed the time signature.
If you're a skilled singer, read the phrases and know where to breathe. For instance, you should not phrase it "oh say does that star-spangled" *BREATHE* "banner yet wave." Bad singer. Learn breath control.
And for the less experienced singers, all I ask is they stay around the same key they started in. Just a whole step or so. Being nervous makes you get tight and go sharp, and I get that. But try and keep the pitch climbing under control.