There is a misconception that polymer parts (it's not plastic) are not as good as metal. The parts in the 92FS that appear to be polymer are mostly polymer coated metal parts. There are a few reasons that Beretta has done this. The smaller amount of metal requred to produce the polymer coated parts saves considerably on raw materials and machining. This translates to a great savings to Beretta, and in turn, the end user (Consumers, US DoD, etc.). Beretta didn't just do it as something to save money, and researched it at great length prior to introducing these newer parts. They found that there are some advantages to the polymer coated parts. Polymer (by its very nature) is self-lubricating, and they don't wear on other components as much. An added benefit is that there is no finish to wear off the polymer.
Regarding the guide rod, as this is a sore spot with some people... The M9 contract had a lot to do with this one. One of the problems with the metal guide rods is that if they bend even a little for any reason, it can inhibit the slide's action (slow slide or binding). The polymer is not only flexible, but self-lubricating so even if it's bent, it goes back into position and keeps working.The fluting adds strength to the design, while allowing the spring and slide to actuate more freely if there is debris. All the guide rod is for is a guide for the spring, but if the slide binds on it, it's a problem. Thus, polymer.
List of possible Polymer Parts (most parts listed here, on new pistols other than Inox, will be polymer coated)
Trigger
Left Safety Lever
Mag Catch/Release Button
Hammer Spring Cap (if has a lanyard loop)
Recoil Spring Guide Rod
Grips