I think plastic can be great if done right.
Apple almost brainwashed iPhone users to hate plastic when most phones have been built on the same material for years. Even the iPhone 3's were made of plastic partially. Metal would block our signal and would be more rigid internally while plastic can absorb shocks better.
Exactly! Everyone hates plastic phones, but that's what the 3G and 3GS were made of. It's more durable than glass and is great for reception. To be honest though, I think the original iPhone had the best design, a mix of metal and plastic.