I've seen this episode of Modern Marvels, and it sounded like a good idea at the time. Unfortunately, even if you break things down to their constituent elements, some of those elements are toxic by themselves, like lead or arsenic. There are only a handful of small plasma conversion plants around the world (Japan, Canada, England, a few others). Several have been shut down because they were emitting pollution into the air. The "obsidian-like" slag might release pollutants if it's exposed to water in a landfill or roadway; whether it does or not depends on what materials (elements) went into it. Another problem is that "wet" wastes like household trash require more energy to "burn" in a plasma than they produce. So then you have to supply a lot of extra electrical power, which is expensive. On the other hand, plasma arc conversion would be worthwhile for things like hospital waste, since the high temperature will destroy any dangerous bacteria or viruses. Normal hospital wastes don't contain heavy metals like lead or arsenic. Several other wastes like plastics, paper, and tires would also be good choices for plasma conversion, and would keep them out of landfills. It appears that plasma conversion is a useful tool, but only if you hire people to carefully sort through the trash--which brings up another fine Discovery Channel show: Dirty Jobs.

Well, it can't generate more heat by creating these waste products than it takes to power the thing in the first place, so it's extremely costly in terms of energy usage. This is why it's not used more around the globe. It also could not get rid of any waste that is dangerous, containing toxic elements such as lead or arsenic, nor would it destroy radioactive elements. It would destroy any toxic compounds, however. As such it is an extremely costly way of getting rid of chemical waste, although at the same time it is ultimately effective. I'm not sure if it would be cheaper to just load the waste into a rocket and shoot it into the sun (which would do pretty much the same thing).