In a word? Spectacle. In TF1, every transformation was pretty much a 'Holy Crap!' moment. The robots were always larger than life, and they felt important. For once, I applauded the use of slow motion, such as in the Optimus/Bonecrusher tangle, or Ironhide doing the double jump; it allowed me to go "Holy shit! These are freakin' giant robots!". In ROTF, the robots are just... there. The first 'magical' transformation, occurs in the dark, with a black robot, so it just looks like a mess of parts. For the rest of the movie, for us, and the characters, the robots are just taken for granted. Demolisher is meant to feel big, but he doesn't, because he just seems to be running away. The use of Blackout in the opening scene of the first movie worked because it again highlighted that these were giant robots coming to kill us all. In the end, I was left simply not caring that there was a big robot deathmatch going on. Certainly none of the actors seem to have been directed so as to give a crap, they all just accepted it as part of the proceedings. Point in case, Leo. He was always terrified, yes. But not by giant robots. There was never a 'OH MY GOD! IT'S A FREAKIN' ROBOT!' moment, it was just "Ah! They're trying to kill me!". Also, look at the casts' reaction to being kidnapped by Decepticons. They're not scared that they've been abducted by GIANT, MOTHER 'EFFING ROBOTS, they're scared of Sam falling out of a car, and Shia's "We got off on the wrong foot" speech just removes any tension from the scene. So, to sum up, in ROTF, the movie franchise lost its heart and energy.