This is a great example of why design changes should always be justified by measurement rather than simply by artistic opinion. I don't care if you think the new design is slick and modern and responsive and a work of artistic genius. Can you prove that it has a significant effect on whatever is our important business metric?
You may not always have the ability to properly make those measurements... it takes me a week before I can gather enough data to even weakly show the effects of a change, and I run a top 100,000 site.
Also it is much more difficult to measure long term impact. For example, I made a change a couple years ago that quadrupled conversion rates, but irreversibly dampened growth. By the time it was evident that it was -that- change, it was too late to recover.
This stuff is easy to talk about in theory, but real life ain't ceteris paribus.
Required sign ups during checkout is almost certainly a business idea. You'll never find a working designer that wants to include flow-breaking elements like a required registration.
