Fingerprint-resistant 10-inch screen
A display measuring 10.1-inches diagonally seems to be the right balance of usability and portability. After all, it's the dominant screen size for Netbooks, so plenty of laptop users are already familiar with it. Most of these small displays are 1,024x600, but higher-resolution 1,366x768 versions are also available. We could see Apple offering the higher resolution as an extra-cost option.
Full OSX operating system
Our worst-case scenario fear is a closed-loop system with an officially sanctioned "App Store," and severely limited capability to install and run outside applications.
Mobile broadband capability
However, Apple has done a great job of using that technology in the iPhone, so we have high hopes some kind of 3G hardware will be included, at least as an option. Also a possibility: a SIM card slot, something of a rarity (but not totally unheard of) on laptops.
An Intel Core 2 Duo CPU and Nvidia GeForce 9400 graphics
At the same time, many inexpensive laptops (such as this $649 Toshiba Satellite A505) have standard Intel Core 2 Duo processors, just like the MacBook line. Let's hope Apple sticks with that instead of the slower Atom-style CPUs found in Netbooks.
Plenty of ports and connections
A few USB 2.0 ports, an SD card slot, and maybe even an ExpressCard would go a long way toward making the device truly useful (plus, we think Apple has learned its lesson after the drubbing the MacBook Air got over its single USB port).