In "lightbox" mode thumbnails obscure the bottom of the image.
If you want to order more than one copy of a print you have to click the "+" button multiple times, there is no place to select quantity unless you go to the shopping cart - confusing.
Maybe worst of all, if you click "back to gallery" it dumps you back at the top of the gallery even if you were on a photo somewhere in the middle or at the end. So you're stuck scrolling back down through all the photos to try to find the place you left off.
I'm sure there are many more things to hate, these are just my first impressions. At least I have a month before my contract is up to find a new proofing service.