I don't have an iPad (yet), and I don't know if I will buy one ? when it's available in Belgium. But I already know I won't buy a 3G one. I own an iPhone 3GS with a data plan and I use it quite often. Why should I subscribe to a new plan in order to have 3G on the iPad ? That's a non-sense, plus it's still too expensive in Belgium.
If I had an iPad, I wouldn't like to be connected more often than on the iPhone, and non permanently.
So what's the missing feature on the iPad without 3G ?
Internet tethering on the iPad via the iPhone.