This was published recently on io9, and it's worth a look:
Science fiction and fantasy are genres where almost anything can happen — as long as the author can make it seem plausible, and as long as it's part of a good story. But that doesn't mean there are no rules. If anything, the fact that these genres are so wide open mean that there are tons of rules out there, some unspoken and some written in black and white.
In general these rules tend to be caged with statements like 'this is usually done poorly but there is a place for it', but it's still worth reading. I must admit I was confused by the rule 'Women should not write Hard SF' since I didn't realise that was a rule (even an unspoken one). In any case, break the hell out of that one, please.