I wrote earlier that high speed 4G LTE capability is finally a compelling reason, for me at least, to look past the WiFi-only model. In addition, the enhancements to the camera (5 megapixel iSight), iPhoto app, 1080p resolution for videos, Retina Display apps (which will consume more storage) it is becoming clear that the 16GB version may also be inadequate.
My strategy with previous iOS devices had been to buy the entry level configurations. Since, as a developer, I upgrade at each opportunity, I could spend $1000 by spending $500 each year on the iPad 1 and then iPad 2 instead of paying almost $$900 once for a fully loaded iPad 1. Plus I end up with two iPads, one for me and one for the kids! Or I could sell the old one and get about half its value back. That policy served me well. That's why I bought the most affordable house in the nicest neighborhood, for example.
I deviated from that strategy when I couldn't find a 16GB iPhone 4S at launch (even though I had reserved one with BestBuy) so I went with the 32GB model. I'm still running out of space on that, so 32GB is going to be my bare minimum with the new iPad. I'm actually surprised Apple didn't offer 32/64/128GB configurations as they haven't upgraded the iPad storage in years despite upgrading the amount of data it will be working with.