Past experience,
SIRI, no reason for it not to work on any Ipad or Iphone, as proved by the jailbreakers, yet it was limited to the newer models only,
Maps, same again, my jailbroken joke of an iPhone 1 runs maps via jail break, even the 3D bits, sure its not as "smooth" as on the 5 but there is no reason not to offer it as an upgrade to the 3G users unless your reason for not giving them the feature is to attempt to force them to upgrade and buy another device again,
Panorma camera, HDR photograpy, offline reading list for 3gs and earlier,
Right now im sat with a samsung Galaxy S2, a two year old phone, and it has the LATEST build of Android on it, its all singing, all dancing, if it was an iPhone 4 from the same period, id be missing features come the next iOS upgrade
http://support.apple.com/kb/HT5457
check here for a list of things not supported as you upgrade, software features being left out of older models that could be easily included, if it was not for the greed of forcing obsolescence...
If apple did not do this, then the new Models of the iPhone would have NO upgrades worth getting a new phone for, the iPhone fives ONLY new feature is a slightly larger screen, everything else the 3, 3GS, 4 and 4S could do if apple didn't cripple them via iOS update abandonment.
its whats made me move to android, im sick of a company leaving out features from a software update to force me to buy a new phone every 8 months, ill keep my Galaxy S2 for a few more years yet and save a boat load on carrier bills, but still have all the features of the latest android OS.