My DD experienced no downsides directly related to skipping 3rd, except that her school quite foolishly segregated children by grade level, so she could no longer enjoy the company of her previous friends. It was very much a short-term problem. The benefits far outweigh the negatives.
One special consideration for boys is that so much of their interaction, positive and negative, is very physical. For social development reasons, therefore, I would caution any parents to think carefully about a grade skip if their boy is already small for his cohort. If he can at least be reasonably expected to hold his own today with the older cohort, and family history suggests that it is unlikely to change, then when all other considerations are good, go for it.
