In the case of Meso-America, bountiful sources of obsidian negated the necessity of metals for tools and weaponry. Obsidian takes, and holds a better edge than either copper, or bronze, so these were not developed by the local peoples.
In the case of South America, there was some usage of copper in weaponry prior to the arrival of the Spanish, and had they not arrived when they did, bronze, and perhaps even iron would have eventually been utilized. We will never know for certain, though, as the natural progression was interrupted.
The civilizations of Meso and South America during the 1500's AD was closely equivalent to Sumer, and early Egypt, and had they reamined isolated would have progressed, and expanded. The nomadic, and semi-nomadic tribal cultures of North America were essentially doomed any way you care to look at it. If it were not the Europeans, it would have been the Islamics, or Asians, or even the civilizations to their south which would have displaced them, simply because they could not effectively hold the resources they posessed with the manpower and technology available to them...
the infowarrior
Better than some steels also. Some surgeons use obsidian blades on their scalpels because they are sharper than steel blades. Fact.