For US passport holders, this sucks. Whereas adding pages to an existing passport is currently a 20-minute errand at the US Citizen Services offices, it's now about to become the same hassle that citizens of other countries including the UK must accept.
Boo hoo.
Fifty-two pages provides quite a bit of room for entry and exit stamps and visas, but for frequent travelers, it means applying for a new passport much sooner than the 10-year validity permits.
Boo hoo hoo.