I feel like America's biggest weakness is a generally sedentary, obese, wealthy population who desire comfort and convenience above all else. This sort of culture does not breed warriors. I'm a fairly big, strong, athletic guy (was a sprinter in college, not a distance guy)....but in all honesty, I live a pretty comfortable life, and I'm not sure how I'd fair in a worldwide, drawn-out battle to the death. I like to think I'd be tough if I had to be, but I've never been faced with this sort of hardship and I honestly don't know how I'd handle it.
Second, our collective arrogance is a huge disadvantage. Underestimating our enemies would likely bite us in the rear big time.
Furthermore, I question our ability to operate cooperatively on a large scale. In our society we have a very individualistic mindset. I'm sure we'd come together a little better when faced with war, but a country like China already has a 1-up on us in that regard.
Our biggest advantage would be our wealth. We have a ton of food and clean water. And from a survival standpoint, obesity could actually be a good thing. Plenty of fat to burn for fuel when food needs to be rationed.
Our size and strength would also be an advantage. Despite not being all that experienced in hand-to-hand combat, I'm 6'2"/200 lbs, I can bench press more than my own body weight easily, squat 300+ pounds, and can run 100m in under 12 seconds. Not exactly a specimen of greatness, but hand-to-hand vs. a 5'6"/150lb Chinese man? I like my chances. Although I'm just not sure I've got it in me to beat a man to death with my bare hands.
Anyways, my hunch is that China's numbers would just be too great to outmatch. I also think that, man-to-man, Russia would be a tough match for us. They have a smaller population, but Russians are big and I'd wager they are generally a much tougher nation.