What's usually going on in a "hot" military operations area (in the US)? Like many people in aviation, I've been around pilots who've flown in or through the edge of a Military Operations Area (MOA) while operating under visual flight rules. Usually they ask ATC if the MOA is hot, and sometimes the answer actually is "yes".  In that case, the pilots stay clear for safety.  My question is, what's usually going on in the MOA at the time and how bad would it be to enter it in VFR?

There are few reputable sources on what typical operations are in military operating areas.  This Global Air blog is the best explanation I could find, although this post isn't a bad source.  Those sources seem to err on the side of exaggerated caution though, describing "lights off" operations, practice dogfights, and 500 KIAS speeds.  Are those kinds of operations daily, or evenly weekly, occurences in military operating areas?  Where on the spectrum from the mundane "practicing approaches" to dangerous "simulated high speed dogfight operating heads-down" do most military flights lie? Are operations consistent or do they vary from place to place and month to month?

Background information for those outside the US:  MOA's are defined in the Aeronautical Information Manual (AIM), paragraph 3-4-5, as well defined airspaces near military bases "for the purpose of separating certain military training activities from IFR traffic."  They are different from restricted zones with particular defensive significance or bomb and missile activity.
Military pilot here.

I can certainly understand why there's confusion surrounding MOA's, having been a Private Pilot long before I flew for the military, and I'm glad you asked the question.

The second post you referenced is exactly right. It is extremely common (at least in our training areas) to have  dogfighting and other tactical training with airspeeds above 500 KCAS, and altitude changes of thousands of feet in seconds. No one uses a MOA for instrument approaches. This type of flying occurs multiple times per day, and it's extremely frustrating when civilian aircraft fly through the airspace without talking to ATC. We generally don't have a lot of time to work in the area, so we have to maximize the training we get. Stopping a fight/intercept/practice bomb run because a Cessna wanted to transit the airspace generally results in a reset of the entire scenario and wastes countless dollars and training opportunities. 

If there's an emergency we will, of course, understand and please do whatever keeps you safe. Short of that, please, please get flight following, and do not fly through a MOA.