Positive
He connected the 2 worlds for the first time.
He opened up a whole new world of sea travel.
Actually, both of those are negative if you look at the viewpoint of the people that were already there.
Negative:
He saw the natives as slave opportunities.
He stole credit and reward for being the first to spot land. (from a lower crew-member.
He opened the path to the virtual annihilation of all the people who knew the "Americas" as home.
He led to the slaughter of millions and didn't even know where he was at.