Well we wouldn't have the rumors that Isabella's sons had different fathers. But other then that not much.
I don't he could marry Ferdinand of aragon!
I've actually been thinking about this lately. If she were born a boy, there wouldn't be Carlist Wars, which is good. However, as he position as heir would be undisputed, it's unlikely her mother would have required the support of the liberals against the Carlist opponents of Isabella. So, it's likely that absolutism in Spain would continue, although not as oppressive as during the Década Ominosa. Perhaps Spain has its own version of the 1848 revolutions?