Nope, you've got it mixed up.
The antenna is still external but the back now connects the antenna bands.
If the back was solid metal it would bridge (or short) the two bands just like your hand did with the iPhone 4.
The plastic inserts are there to break up the back and stop it from bridging the bands.
And to the people expecting something radically different.
Come on!
This is supposedly one of Jobs' last projects.
Jobs, the guy that bought the exact same car in the exact same color every six months for years!