Bigger screen, LTE, faster processor, improved battery life, also a better screen as in addition to the size it is also clearer and has less reflections. Not to mention thinner and lighter, but that just depends on how important those qualities are to you.
Those are all the things you would miss out on if you just kept an iPhone 4S and installed iOS 6.
Without trying to be facetious, most of those features don't matter to me (Maybe I'm getting old, I'm 28 and consider myself geeky).
LTE in the UK is non-existent and won't be widespread for a couple of years. The bigger screen is nice and all, but still won't make it a good medium for watching video, and the screen glare has rarely been an issue for me, so it's a solution to a problem that didn't exist.
For me, it's a very easy decision to make, and I know of a number of people who were waiting for the iPhone 5 announcement who are now considering other phones.
I have no doubt whatsoever that the iPhone 5 is a fantastic phone in it's own right. I'm just not sure there's a compelling argument to make it 'the' phone to have in the way that it always has been, beyond the shiny cool factor, which is something that cannot be relied upon.