[I made this comment earlier under another thread but I feel it deserves its own thread.]
I like the new W10 overall and don't mind using Photos as a default image viewer except one issue:
IT HAS A FLAW IN RENDERING PICTURE SHARPNESS.
This bug has been there since Windows 8. Basically it often (but not always) shows sharp image as blurry as shown below. On the left is a partial screen capture of a photo as displayed by Windows Photo Viewer (same sharpness in other programs like Paint, Photoshop, etc). On the right is how Photos app displays it. It's obviously quite blurry compared to the left.
Note that this was at 100% crop. In other words, there was no resolution scaling involved at all (scaling step could potentially influence sharpness somewhat depending on scaling algorithm) so it should have been very straightforward to display the image at 1:1 mapping and sharpness should be same on both programs.
Like I said, not all photos are subject to this problem. So I dug in a little to see if there's a pattern I could spot. And I think I found it. This blurriness occurs when image resolution in either dimension is odd numbered. For example, my 1716x1080 images are rendered fine in Photos while 1631x1080 image (shown above) is not because 1631 is an odd number. This odd-is-blurry, even-is-sharp pattern is observed throughout my samples. And when I tweaked the above image slightly in Lightroom to change the resolution to 1630x1080, it looks fine in Photos.
I hope this information reaches the dev team and they can do something about this issue.
Images in question are in JPEG format. I do not know if other formats are similarly affected.
EDIT: It's been shown that at least JPEG and PNG images are affected in the same way.
UPDATE (2015/10/01): A few weeks back, Microsoft released an update that fixes the blurriness issue for the sample photos discussed early in this thread. Kudos to MS for responding to us! But upon further testing I found there are still cases where the issue persists (see pg 3). MS engineers are looking at the new samples I have provided.