Chemistry in the band is very important. I remember a qoute of Bono's, when u2 were a band of 19 year olds, he said "I really feel we are destined to become one of the great bands, we have a chemistry. The Beatles had it, the sotnes have it, and u2 have it." Now personally, if anyone actually said that to me I would want to see them fail simply to teach them right for being so ****y, but it shows that if there is a great chemistry, you know your band will be good.
To be succesful, I think that you need to love music, completely and unconditionally. Music comes first (sometimes unfortunatle to the detriment of some relationships). If it doesn't, you can still be a good band, but not that extra bit that seperates the amateurs from the rock gods.
FInally I think that (and many might disagree) for a rock musician, you need to have a desrie to be famous, to be the best. I don't believe that seriously there is any band that at least initially didn't have a yearning for fame. It might be selfish, it might be ruthless, and it is certainly arrogant, but I think that it is needed. You look at most of the rock greats, and by and by they have a passion to be noticed. I don't think this is true in all music, but I believe it is true of rock, that is so wrapped up in the charisma of the performers, and the drive to suceed whatever the cost.