I like the "premium" feel of the glass, but as mentioned above, a liquid metal back would be awesome. It would also be nice to not have to worry about the front AND the back cracking.
I want it like the MBP. Whats ever that is. I love that feel and its very scratch resistant. Aluminum I guess it is.
Glass. Nothing looks worse that the aluminum back of my iPod with all of its scratches from normal, day-to-day use.