Most Current motherboards usually convert 12V to Vcore and Vnoth_bridge. The reason is simple: 12V allows higher efficiency for the DC-to-DC regulators. The regulators themselves are powered off the 5V rail, but some can use the 12V rail also. I highly doubt any PC has regulators off 3.3V. PCI-Express has 3.3V and 12V (3.3V for logic level and 12V for power).
You can doubt if you like, but they do, particularly for memory. It is not very significant difference in efficiency using 12V for this purpose, while it is possible, it is just as viable to use another rail, particularly so the board isn't putting an undue burden on only one rail which tends to cause problems for "most" PSU because "most" don't have truely independent regulation.
Most new motherboards look at PWR_OK and other signals directly from the PSU unit. If these are not within 10% of nominal, then your system will not boot.
That is overgeneralized unto the point of being incorrect. You can take some new boards and just short the PS-On and Gnd and get a board running even without Pwr-Ok connected.
Another indicator would be a ton of hot air out of the PSU - the silver box will do everything it can to get that 3.3V up to at least 3.0V.
Absolutely NOT. For one, the 5V rail and with newer PSU, 12V rails are those weighted to obtain the regulation, the 3.3V is a derivative on most, and even with independent rails the 3.3V is not the priority feedback rail, the PSU will let that rail drop, but not down to 2V.
It would not create a ton of hot air either, this has no backing at all. If it only increased one rail on an independant rail system it would be a fairly trival increase in total power consumption, or if non-independant rails, there would be a corresponding increase in voltage on the 5V rail, IF the PSU had enough bias to 3.3V in the feedback loop, which the majority do not. No matter what happened (except for a failure) the condition would not suddenly start producing a lot more heat.
Also keep in mind that the silver box is designed to give the proper voltage under load - it is usually measured with a few Amps of current to make sure it is in regulation. If you plug in a PSU and measure no-load - do not be surprised if the voltages are all over the place.
Here we agree.