Obviously, the laptop form factor is missing, but it would be...interesting if the iPad, iPhone, and MacBook were all pretty much the same compute device with different form factors and different battery sizes and possibly different storage sizes. Honestly, the form factor and user experience are going to be the biggest user-visible differences, and those are also the things Apple seems to care the most about (even if they don't consistently get it right!)

Also, why would you want a single physical device anyway? Because hardware is expensive? Sure, maybe. Because you want to keep all your data in one place? That's the purpose of iCloud. Because you don't trust the cloud and want to keep all of your data physically on the same device and still access it from multiple form factors? That's a small fringe of the market that probably wouldn't buy Apple products anyway.