Screenwriter Roberto Orci has some answers to the many 'Transformers: Revenge of the Fallen' Questions.
Firstshowing has the article. Below is an excerpt though dealing with the difference in themes between the two films
"The first movie is all about the mystery and reveal of the Transformers as a concept and a species, which means we had to hide them a lot in the first hour. In the second movie, now that everyone believes in the universe, we can jump right into the world of the Transformers… The hardest aspect is that you lose the element of surprise. We got away with a lot in the first movies because for a long time, no one thought it would work, or that it was just a silly cartoon, and simply proving that wrong is all we had to do. The first movie was meant to be somewhat of a mystery. For the sequel, we need an entirely different structure and paradigm for the movie, and we can't rely on it being the first."
for the full article