The moment rumors of a new Dock port emerged, many expressed hope it would feature a MagSafe-style connector, like Apple's MacBook line of laptops. Symmetrical, it could be plugged in without worrying about up or down, and magnetically coupled for better ease of use. Others dream of ThunderBolt (sadly, that requires PCI architecture, which iOS currently does not have). Others simply wanted to make sure all their old Dock accessories would still be compatible and still work.
Compatibility is an easier topic to look into, if only slightly...
The Dock probably doesn't need the unassigned pins anymore. It probably doesn't need all those grounds. Given Apple's fearless aggression when it comes to dropping aging technology, it probably doesn't need the composite and component video pins, or the serial pins.
If Apple really wanted to, they could probably cut all the way down to 4 pins of pure USB (2 data, power, ground). If they did that, however, they could just go with a micro-USB connector, put a smile on the EU's face, and be done with it.
But they're not. They're going with a Dock connector, only newer and smaller. It's tempting to assume the worst, that Apple wouldn't go to micro-USB simply to maintain proprietary control over their Dock connector licensing program. We could also assume the best, however, Apple might be doing what they did with the original Dock -- making a single connector that can do multiple things in the most compact packaging possible. Likely there's elements of both at play.
Multiple things, however, means more pins. TechCrunch claims they've heard 19 pins. iLounge heard 8. 9to5Mac found strings in iOS 6 for 9 pins.
4 USB pins, plus 4 HDMI (2 data, 2 clock) pins would come out to 8 pins total. Add a proprietary detector pin, and you have 9. (There'd still be a 3.5mm jack for legacy audio.)
Take those 9, however, and add back the 4 audio Line pins (left and right, in and out) for and you have 13. If USB 3.3v and 5v are kept separately as they are now, that's 14. 2 serial pins, 3 composite and component pins (video out + Pb, Y, and Pr), and that could be a way to reach 19.
Obviously, for accessory makers and current and past accessory owners alike, a 19 pin Dock would offer a far more options. With the adapter we learned about in July, it would allow backwards compatibility for the widest range of existing accessories, including the aging video standards.
Unless Apple makes a very elaborate, and very expensive adapter or set of adapters -- which based on past history is highly unlikely -- an 8 or 9 pin Dock would greatly reduce compatibility with current and past accessories. Old cables could charge and transfer USB and HDMI data, but not much else.
Bottom line, space will be at a premium in the iPhone 5, and given the way Apple shoves old connectors aside, even their own FireWire, to make room for the future, it wouldn't be surprising if the newer, smaller Dock connector goes with the fewest pins possible.
