The clickwheel and the digital compass are PDA features that make the biggest difference to me. The Blackberry's clickwheel interface means that you can operate all of it's functions using just one hand and most of them using only your thumb. As a platform for the clearly defined productivity applications that make up the core of their value proposition to business users, this simple and elegant design decision makes the Blackberry a standout. The one handed interface has the added benefit that you drop the device a lot less often because you have a another hand free to deal with all the other other exigencies that crop up when you are mobile. Apparently the RIM engineers also spent a lot of time making the Blackberry rugged, because I have dropped mine over twenty times and it has not only never broken, but hasn't even rebooted in response to the trauma. Your mileage may vary, but one drop meant sudden death for the iPhone.
The iPhone, like the Blackberry is also tightly integrated but with a bit broader scope and consequently a less less elegantly designed manual interface to accommodate the range of applications. The digital compass is a relatively new feature that I used on the iPhone3GS last weekend when I went “geo-caching” with the family. The geo-caching application was no great leap forward because it uses the same technology as a competent car GPS device. It dawned on me, however, that until the 3GS version, the iPhone could only deliver orientation information via the accelerometers or coordinate information via the assisted GPS, but had no way of integrating them into something useful.
The compass adds that crucial link that lets data services know both where you are on the face of the earth and which way you are facing. Changing the location framework from fixed absolute Cartesian coordinates to the user POV enables some profoundly powerful applications that people might find invaluable. Just like car GPS devices turn the map so that “forward” is always “up”, so could PDAs which employ the accelerometers, the GPS and the compass orient the display to accommodate the orientation of the user. But these applications will have to be imagined to be developed and here is where the ergonomic Blackberry, and well integrated iPhone are going to have to cede the high ground to Android. That's because Google has the technology to make something useful out of this capability. Like superimposing Google Earth data on top of a picture taken with a camera.
From here I can imagine all kinds of interesting applications (both productivity and entertainment) which combine the absolutely positioning of GPS with the absolute orientation information of something like a Wiimote. I believe that Google with it's portfolio of technologies and user-base of developers is best positioned to deliver that value.