draythomp:
What I'm looking for is why there is so much bad press about messing with the mains.
IMHO it's the lawyers talking (present company excepted). But not without some good reason. Kind of like those car commercials (do not attempt, closed course, professional driver). How does a Kill-A-Watt do its thing? Back in the day, most all of my projects were powered via step-down transformers. And certainly there is a lot of safety and peace of mind there. Now I use these wall warts that have switching supplies in them, I don't think there's an transformer involved to provide isolation from the mains.
Certainly there are folks out there that might try something that they don't completely understand, and therein lies much of the danger. So first it's a matter of understanding very well what one is dealing with. From there, I can think of a couple major concerns. There may be more.
(1) Coming into contact with a mains-connected circuit always has the possibility of delivering a dangerous, potentially lethal shock. I sometimes have the bad habit of removing or adding components on a breadboard without cutting the power. With a 5V supply, there's not much danger to my person. Of course there is danger to my project. I could short something out and destroy some component. Depending on the component, it might just not be a good idea. But I usually think I know what I'm doing, so if I just want to change a red LED to yellow, I'll go ahead and do it. I wouldn't do it with an MCU. And I'd never think of doing such a thing if there was any mains connection whatsoever.
(2) The less isolation from the mains, the higher risk there is of some transient (lightning strike perhaps being a worst-case scenario) affecting the circuit, destroying it, or even starting a fire. I have heard it said that there may be insurance liabilities with homebrew mains-connected devices. So some thought to failure modes and proper design to either prevent a problem or to fail in a safe way is in order. This is a big part of what UL ratings, etc., are about.
So of course it can be done, and in fact it is done regularly, because all of those gadgets and appliances we all have plugged in are examples. But they are all designed in part by engineers who specialize in this very area. And yes, like a lot of things these days, the warnings are somewhat over-zealous, in a (perhaps futile) attempt to protect those that don't know that they don't know from themselves. (Lawrence Bullis had some interesting ideas on that score.) Personally, I wouldn't hesitate to experiment at bit, but at the same time I couldn't recommend it to anyone if I didn't know that they understood what they were doing. The risks (above, and probably others) need to be given some thought and mitigated appropriately.
I'm dating myself (I barely remember these), but consider the "All-American Five" radio.