Revolution! Most of the former Spanish controlled areas in Central and South America broke away from the Mother Country Spain by the 1830's, as her economic and political influence waned. European colonization of the New World followed this pattern of colonial establishment, development, and ultimate rebellion.
Horses! The Conquistadors reintroduced the extinct horse to the Americas, and they have been a vital part of our life and culture ever since. Columbus brought horses with him in 1493, and then Cortez brought horses in 1519; many horses either escaped or were stolen, and they spread until we think of them as native. In fact, the only real wild horse left is a Mongolian breed called Przewalski's Horse. Think of old movies and TV shows, think about racing and show horses, think about The Black Stallion! The Spanish brought horses to America, and everyone was better off for it.
It is said that, on the positive side, South Americans got a language. It seems questionably positive as the acquisition was at the expense of the loss of other languages. It seems excessive to compare this gain (which may have been as undesired as European diseases) to the European gain of dominance, wealth and slave markets.
People in South and Central America wouldn't be living the same way as the ancient Mayas, Aztecs, and Incas. Indian civilization saw enormous change over time as well, and no doubt would have continued to do so without European intervention. And I would hesitate to describe South American "democracy," to the extent that it has ever existed, as a Western contribution. It seems to me that the "positives" would depend on who you asked, but I'm not sure that the polities within the Aztec or Inca empires would have viewed Spanish colonialism as an unmixed blessing. In the long term, though, it's hard to argue, unless from an unabashedly Europhilic point of view (like, frankly, that of Niall Ferguson) that native peoples' experiences with Europeans were anything but a net negative.
For a fuller discussion of this topic, you may want to look at Niall Ferguson's recent book The West and the Rest. He suggests that over a course of many centuries, the people of South America have ultimately benefited from such such "western" achievements as democracy, modern medicine, free-market, economies, etc. Would it really be good for people today be living as the ancient Aztecs, Mayas, and Incans lived? This is a question worth pondering, although no one can or should deny the brutality of the Spanish invaders.
The negative is going to be easier to spot here. The Spanish essentially came in and subjugated every native population they came in contact with. They destroyed native cultures and assimilated the natives into their own culture at the lowest levels. The only good I can think of is that they destroyed the Aztec religion, which was pretty brutal in its own right.