I find no one here has suggested my reason for upgrading. But more on that later...
Re- SD card slot. For those who would like this, fine but were you aware you can have an SD card slot now, works even with ipad1 ! As Rocky stated, the SD card implementation is what is different and as such is quite specific as to what you can do with it. I regularly transfer images with the adapter from my camcorder. I did a PDF file once too. But it definitely is app specific.
Personally, I feel the way ipad works is one of the most difficult and awkward things for a PC mindset to get used to. I'm not suggesting Apple change that, just that compared to an Android device, not being able to freely manipulate files is something that still makes ipad awkward for me. Unfortunately this seems more of an iOS issue than a ipad hardware issue ( for me).
The quickest reason for me to want to upgrade would be if my battery stopped working or my current iPad broke.
I'm currently interested in 3D so having a glasses free 3D screen and be able to play 3D SBS or Frame compacting would be an incentive.
Screen bright enough to see easily in bright sunlight.
Nothing else I can think of is compelling enough to want to upgrade.
I really have no desire for the camera features and would probably never use it regardless of quality. I saw one person so far ( at Disney) using an ipad2 camera and he was obnoxiously annoying with that big view blocker in the crowd. I'm glad more people don't use the ipad2 camera in public.