Realistically you have to think about what it can offer that that the current model doesn't.
The obvious improvements/additions will be:
- Faster Processor
- Resolution
- Camera
- Siri
Personally, I've owned the iPad2 a year and am yet to see it struggle to do anything. I don't have any issues with the resolution, though of course a new display is nice. Even if they improve the camera, the iPad isn't a great device for taking Photos with, I only really use my iPhone or a proper camera. Siri was fun for about 5 minutes, but I barely use it anymore.
What is more likely to make somebody want to upgrade is the unexpected 'wow factor', be that a feature of the device, or an App/Service which is device specific. I think they are bound to have something which nobody has predicted to make it really sought after by those who already own iPads... otherwise, I personally don't think I would feel the need to upgrade based on the likely changes.