Much more orphans. If you're using POW, the delay in the block relays would be much more significant than the block interval.
If the number of required confirmations remains the same, the security of the merchant accepting the transactions would decreases significantly. As compared to before, less resources is required to attack the chain and double spend your own transaction. I can't think of any significant advantage to lowering the block time to that amount.
Instead of a strict targeted block time, would it be possible to adjust difficulty partly based on orphan rate? So that as orphan rate increases, difficulty would also adjust upward, increasing block time (and lowering orphan rate).