I will definitely be upgrading to the new iPad Air 2, probably at least 128g. But I am a power user.
I use my iPad 3 for absolutely everything, I have even given up the notebook. My dad has the iPad Air and the difference is chronic, ney intergalactic,...well you know what I mean. The iPad Air vs the iPad 3 shows more than a 5 x performance jump!!!
I suspect that the reason people are not interested in upgrading from their iPad 3's is that they are not using it for anything too intensive. You will really see a big difference when editing complex/ large pages docs, the same goes for keynote presentations.
Where you will also notice a big difference is in fast multitasking.
As you know BioShock was just released for iOS, well guess what the iPad 3 is not supported....the first time I have seen my iPad 3 not supported for something and so.....it begins. Can you blame Apple absolutely not, times have changed.
With that being said, apart from sentimental reasons, I can't imagine not wanting to upgrade from an iPad 1 or 2, with what's coming out.
But that's just me,