On difficulty, some instruments are harder to learn then others, but all of them are equally hard to master. Violen is fretless and therefore harder to learn then guitar. The reason I think that there aren't any real good female guitarists is because guitar is a masculine instrument. It is aggressive, passionate, controlling, strong, ect. Females tend to be better at instruments that fit their stereotype. A flute, for example, is a feminine instrument because it is beautiful, passive (usually), ect. Thats my theory anyways. I'll give the feminists some credit though, it is somewhat cultural as well. I mean, how many women are encouraged to pick up guitar?