1. Retina Display - Would have been cool, but as stated this would have driven availability and price thru the roof. One article stated that the retina display was going to cost approx $75 dollars more than the current display for the part alone. Add in additional R&D, profit margin, etc and you are probably looking at a price bump of $150-200 to the consumer.
2. SD Card Slot. Would have been cool, but not surprised. Right now we can't get to the file system, so including this would have been for very limited usage (probably just copying photos).
3. Thunderbolt really? Have you seen the size of the thunderbolt chip inside the Macbook pros? Where would apple have put it?!?! This would have increased the thickness and/or the overall size considerably. Besides, the current port size is thicker than the thickness of the new iPAD. We need to 1st have devices that utilize it and then they would need to create a "micro-thunderbolt" connector. Besides, wouldn't being tethered to a wire defeat the portability-on-the-go aspect of the iPad?
4. NFC makes sense on cell phones for three reasons: 1. You always have it with you, 2. It is pocketable (like your wallet) and 3. it has an always on data connection. iPads are none of these (with the exception of the 3g iPad). Frankly, whipping out my 10" tablet to pay for my latte seems stupid. (yes yes I know NFC will be used for other things eventually, but for now that's what is it is planned mainly to be used for).
5. 4g. I could see the value. With that said, last I checked, 4G sucked power like nobody's business, and the coverage is poor (not in my neck of the woods).
I think eventually we will see most of these added, but only when they become reasonable to add and/or deemed to be an industry necessity.