In Mac OS X, I like seeing all of my email on one page, and with a quick click I can look at any email account segregated from the others -- easy peezy.
You would have thought that the famous Apple GUI designers would apply the same logic to the iPhone, but for more than two years I've had to really struggle to see what is coming in on 3 different accounts. I'm tapping more than the Titanic's radio operator just to see what email I have.
I hope the fix comes soon, and that it winds up on the iPad as well. The lack of a unified or universal mailbox is one of those strange Apple decisions that is hard to understand. The lack of cut, copy, and paste was another seemingly bone-headed choice that Apple finally got right. Now Microsoft seems to be drinking the same kool-aid with its upcoming Windows Phone 7 OS. The company says no copy/paste will be in the phone OS, and they're telling users they don't really need it anyway. Sure.
Apple makes a lot of great choices and we love Apple products, but every so often they seem to miss a big feature. After a lot of bad feelings from customers, they finally make it right. Usually. Sometimes. Meanwhile, I'm waiting for that unified inbox, and switching frantically between mail accounts. Grrrrr.
Can you think of some other big missing items? A camera in the iPad comes to mind...