After watching Due Date, it's not hard to imagine the pitch meeting where whoever came up with this script made his case: it's The Odd Couple meets The Hangover. That sums up the movie and if you're going to hold it up that standard, then where each of those movies are great in their own rights, Due Date does not rise up to those standards. The humor is neither uncomfortable enough nor is it nearly as outrageous as it needs to be. It merely ends up as a tepid dilution of both films.
As for the acting, neither actor is doing anything new or exciting. Robert Downey Jr. could easily be playing Tony Stark or Sherlock Holmes. We don't see the nuanced and artful performances we can see from him, as we did in Chaplin. And I don't think I've ever seen Zach Galifinakis really give a performance that is very different from what we've seen in his stand-up, and there is no difference here. Neither actor is really acting very far out of the pocket.
I was hoping for more from this movie because I do like both actors, but sadly it just did not deliver.