I do not want a tapered back, after having the 4 I really really like the fact it will sit flat on the table (that's more important to me than how it feels in the hand. Honestly, the only time I notice is if I hold a 3G in my hand before I hold the 4. Otherwise it is not something I even notice. But I do enjoy my phone not rocking when I'm using it when it's sitting on a table).
I personally think Carbon Fiber would be nice. Course I'm not knowledgeable on the strengths and weaknesses (cause I do know it has weaknesses but I can't remember what) but I would think Carbon Fiber would be durable enough, it would look decent (sorry aluminum lovers but I hate the look of the iPhone with an aluminum back, blech), it wouldn't block signal (another problem of a metal back), and the only thing I can foresee being bad is it may not be cheap enough to use.
Other than that, I think I prefer the glass over the plastic. If you take care of the glass (don't drop, put a case on it), it will stay nice so at least you have a chance of it staying nice. Where as I find Apple's history with plastic is that it will inevitabley crack (between my old Macbook which every single person I knew with that macbook ended up with cracks on the palm area, even one guy who had his top case replaced the top case cracked again, and my iphone). And yes, I realize drops happen and I agree that a phone should be able to take a drop. But I find my 4 has done fine with a case on it (haven't cracked/chipped it yet and trust me, it's been dropped).
Third choice would be... I have no idea. Some one had an article to some material that I thought sounded promising, glass metal:
http://www.sciencedaily.com/releases...0512150815.htm
Honestly, the third would probably be the best choice.