First and foremost. McCain has a likely chance to die within his first 4 years. He's fought cancer, had heart attacks, and is in his 70's. he also has(supposed) mental trauma from Vietnam as well as being a POW in the Vietcong camp.
Second, people have questioned whether or not she has abused her power as mayor, and as governor. She likely has as the latter, far less likely as the former.
And finally, as quoted by Matt Damon in a video criticizing her, "because she'll have the nuclear codes". We don't know if she'll use them or not, or if it will just be Iran, Russia, North Korea, or all of them. Hell, she might even send on Venezuela's way for good measure.
That's why many people are making a big deal, she's a relatively unknown politician, and people don't know what she will do with the power if McCain gets elected, and bites it during his first term.
And lance, let it go. she lost, big whoop. at worst, it'll be 2016, at best(counting on how you look at McCain winning) it'll be 2012. Either way, the campaign is over for Senator Clinton. And BTW, that ad told nothing but the truth, as opposed to the McCain ads.