They all look like this. Spotlighting at the top.
My 6 has this primarily in the upper right corner. It is very slight and not enough to return the phone for. It's mostly noticeable when using the Kindle app, but I don't often read on the phone. They don't all have this however if you count some of the ones on display in the Apple store. It's harder to see under the bright lighting and screens turned up to 100% brightness, but some clearly have it and others do not. A lot of the 6 phones in the store appear to have perfect screens - no spotlighting at the top and no yellow tints.
I looked at 3 stores and about 40-50 6 and 6+'s. All had some spotlighting at different degrees. Some near undetectable, some down right ugly.
Mine is like this as well. I started a thread literally a few hours before yours outlining the same issues.
Part of me wishes I had waited a couple more months before I upgraded to the 6 so that these kind of problems had been ironed out.
My 6 has a seemingly darker spot just in the corner where the battery icon is. It's not terribly noticeable at all, so I'm not worrying about replacing it. The rest of the phone is flawless!
Sorry to hear about some of the others ones in here.