My guess would be strength. The iPads are very rigid considering how thin they are. However good design and the right resin could address that.
The big problem here is that many think of plastics as cheap, that isn't always the case. Some of the really impressive structural resins are expensive and not easy to work with in the plant. Apple would have to find the resin with the right mix of structural qualities.
This highlights something else, just because there are many cheap plastic products out there doesn't mean that everything plastic is cheap or of low quality. I tend to suggest that people look at power tools for examples of high quality plastics people ca relate too. By the way I'm not talking Harbor Freight power tools here but rather contractor grade tools.
Originally Posted by liquidsky617
I could envision Apple taking their experience with producing a plastic iPhone and scaling it to the iPad.
Theoretically, lighter form factor. warmer feel (versus cold metal). colours if needed.
Compete with Amazon's Kindle.
Why must the iPad be metal?
Must it be metal? Frankly I'm not sure. Part of the problem might be in the challenge of injection molding a rather flat thin structure and to get that to come out of the die flat. Beyond that some machining would likely be required anyways. I do believe Apple should pursue alternatives though.