Why sucks

I just spend 45 minutes trying to create some funky business card-sized prints of flickr photos on MOO.

Very slow AJAX
The site uses heavy amounts of Ajax to do in-browser selection and cropping. It looks very pretty. But it's ridiculously slow. Their minimum batch is 100 photos, and it takes way way too long to do this. Click. Wait. Drag. Wait. Wait. Wait.
This is especially unforgiveable because a non-Ajax solution would have worked just as well: why not just show me my photos with little checkboxes beside them?

I cannot save my order-in-progress
The most terrible sin of all. After waiting through the process of selecting my 100 photos, I cannot save my work. I must complete the entire order and hope my browser doesn't crash in the interim. When I was suffering through the horrible speed, I wanted to save my order and try out the site in a different browser. But since I cannot save, this is impossible!

Their site crashed on the last step
After all that work (did I mention it took 45 minutes!) I entered my Visa number and clicked the 'confirm' button. And then the site responded with a completely blank page called preview.php. No amount of refreshing and resubmitting the form gave me a proper response. So all my effort is completely wasted, and I don't get any cute printouts. And since I couldn't save my order, if I want to retry I'll need to invest another 45 minutes!

MOO is a really neat idea, but their website is absolutely hateful.
I couldn't agree more with you. Why isn't there a save my order in progress feature when the whole thing is so goddamn slow. I gave up at the cropping screen after 20 minutes. (can't imagine how severe my disappointment would be if my order didn't get through at the last step like in your scenario!)

Moo's a really, really cool idea that's poorly executed.
Also their uploader resizes the artwork on the reverse, even if the artwork is identical. the preview is incorrect so the final cards are cut wrong.