The really frustrating part of the "softness" is in the pic of the goldfinch. Had it been in sharp focus, I think it would have been a magnificent picture. (Note that it is a cropped picture. For the full picture to be displayed on my monitor, it would be reduced to 25% of full size - as were the last eight pictures.
Now, here's a wonder question. I was trying to get a good picture showing the problem to send to Nikon along with the camera. The birds weren't cooperationg so I decided to take a picture of the feeder. It has lots of dark horizontal and vertical lines to focus on (the distance is 15 feet and the lens is at 200mm). While I was waiting, I started fooling with the diopter adjustment and carefully adjusted its focus on the cage of the feeder. Then I shot some pix and when I checked, they seemed considerably sharper than before. (Flowers taken at 2 feet didn't seem to benefit that much from this adjustment)
I saw a diagram of the mirrors and sensors in another model of Nikon (D90, I think) and it showed the focusing sensor on the bottom of the camera and the picture sensor (the "film") on the back. Is it possible that the focusing sensor on the 5100 is on the top, after the diopter adjustment? Does anyone know of a site that has a diagram of the internal layout of the 5100?