The big difference between Sebaceans and Humans is that Sebaceans can't stand hot environments. They go into Heat Delirium which can lead to death. Sebaceans have better eyesight. Their gestation period for pregnancy is considerably longer as well. They also live much longer than humans.
Oh, and it's CRICHTON, people!!
Crais went from original bad guy to ally. One of the best characters in the series. Scorpius is essentially still the Big Bad but it changes all the time. With the exception of the crew of Moya, everyone wants to capture or kill Crichton. Sometimes, even his friends want to kill him.
At the end of the 4th season, the biggest bad appeared to be the Scarrans who are the Sebacean's main enemies and will be a focus in the Peacekeeper Wars. Scorpius is a half Scarran/Sebacean who wants badly to defeat the Scarrans. The problem is that while he is technically a Peacekeeper he really doesn't care if both sides kill each other off - he just wants the Scarrans crushed.
Hopefully, that will give a little more insight into the characters leading into the mini-series. There is much more to all of this than that but it will help.