I've been following Prometheus as closely as I can since I'm really looking forward to it. From what I hear, this movie centers around the 'space jockey's' (remember in the first film, the derelict spacecraft and the fossilised creature sitting in the chair with it's ribs broken outward? Thats the space jockey for those who don't know.) and mankind's first encounter with them. Obviously there's more to it than that as well.
I've always loved 'Alien' and 'Aliens'. I thought that both were brilliant movies. I actually don't mind 'Alien 3' but I recognise that it's not as good as the others. And 'Alien Ressurection' just didn't have the same feel. But, why I have hope for this film is that it's Ridley Scott at the helm and I believe he's been giving a lot of leeway on the decisions about the film. So hopefully none of the studio execs will have been able to make any poor decisions that ruin the film.
The film was originally going to be a straight up prequel to 'Alien', complete with the actual aliens themselves and I think the film was essentially going to lead almost right up to the events of the original. Then those in charge (and again, I think I remember reading this decision was made by Ridley Scott) decided that it was not a good idea to do it as a prequel. So they decided to distance themselves a little from the Alien franchise so this will no longer slot into place with the rest of the films. I think it's being promoted as more of a stand alone film thats exploring some of the other idea's within 'Alien'.