The iOS 6 is a major addition to the iPhone, iPad, and iPod Touch, and fundamentally alters both the design and features. This addition has been made to not just the new iPhone 5, but also available to some older versions as well. Apple has always practiced a form of planned obsolescence that does not allow new iOS updates for all of its previous devices. This is stated to be that the new features are meant to be compatible with the devices that can handle them, yet this generational shift is not usually hardware based. The iOS 6 has also left several devices out of the loop, and at a faster rate than normal this time.
The iOS 6 is not going to be available on first generation iPads, just the iPad 2 and iPad 3. The iPad 2 remains limited as Siri and FaceTime through a 3G connection cannot be supported, but this only affects those who have a 3G connection applied to their iPad. Hearing aid is also missing.
It also does not support any device below the iPhone 3Gs, though it is debatable how compatible it is there either. The iPhone 5 will not support Siri, the turn-by-turn navigation or flyover in Maps, and you cannot use FaceTime with a 3G connection. On the iPhone 3Gs it will not support the Offline Reading List, Siri, shared Photo Stream, hearing aid support, VIP list, or the same maps and FaceTime functions.
The only iPod Touch model that is allowed is the 4th generation, and this may be an indication of its eventual decline as an independent device or a new one will be on the way. The iPod Touch is often left behind in these updates.
This is a much more dramatic exclusion than was previously seen with the iOS 5, and since the lack of shared features on even recent devices is dramatic it may be a testament to the hardware required to use all of it. This will also show a bigger push towards the iPhone 5 as older users are going to be left behind in this firmware evolution.