My favorite thing about Pandora's Star was the aliens. Definately. I've never read anything quite like them, and it was very interesting to see how they evolved. Quite scary, but very interesting
As ever with Hamilton, his space characters are pretty cool. The added amusement of the astronomer on the ship added an important extra dimension to the story, and made it better than the space parts of Night's Dawn, IMHO. Him being uprooted from a rather naive life into such spectacular and public events was another interesting plot line too read. I always tend to like those characters, who get chucked in at the deep end with no idea how to survive...
The other part that I particularly liked was the planet owned by that one person. I'm terrible at remembering names, but was it David someone? Its something that pops up with reasonable regularity in sci fi books, but this one was better than most, I felt.
Very few dislikes, to be honest. I have high hopes for the sequel, Judas Unchained, but it will need to wrap up a few of the less likely plotlines well, otherwise it could just be dissapointing.