Keyboards/synths, and other instruments not seen as often in the metal genre are more commonly used in progressive metal.
It's common for progressive metal acts to have very long song lengths with plenty of complex instrumental sections which give the musicians a lot of room to show off. Odd-time signatures are also are commonly implemented.
Progressive metal lyrics tend to be very cerebral, with elements of fantasy, psychology, and science fiction thrown into the mix. (The sort of lyrics you'd expect from Stephen Hawking or Sigmund Freud if they ever formed a heavy metal band.)
The intelligence is usually shown by the fact that the people that like prog metal can actually spell Dream Theater's name right, as opposed to the people who hate on prog/Dream Theater
Me: "It's spelled Theater you fucking idiot, go wank to some gay Green Day shit."