In the first Iron Man movie, Tony Stark was clever and bold. In the second, he was confused but arrogant. In The Avengers, he truly emerged as a hero amongst heroes, growing up while maintaining his wit and charm. If the trailer is any indicator, Iron Man 3 has Tony Stark turning into a bitter, grumpy man.
The quick one-liners are gone, replaced by depression, vitriol, and selfishness. He is vengeful. His nemesis seems powerful and philosophical. This is nothing like the trailers for previous Marvel movies. In short, it should be exactly what fans have been wanting – a Tony Stark that takes advantage of the range that Robert Downey Jr. brings to the role.
His grumpiness can be seen in these screenshots followed by the trailer itself.