There must be more to it than that, right? Can you fill me in with more details as to how it works, if there are any restrictions on creature type, or anything else?
There must be something.
Basically? No, there isn't more to it than that.
Some explanation would be needed as to why certain monsters fall apart at the slightest scratch, while other seemingly identical monsters are full strength and take many more hits to kill.
That's something that PCs' don't see in-world, obviously. The logic is the one of movies, where you have the protagonist going through ranks of underlings, occasionally taking a hard hit, to reach his mortal enemy, the real target. Like Aragorn against all these Uruk-Hai, fighting his way through to the leader of the Isengard party (your avatar).
That's definitely not something you want to abuse/use all the time. But if you are running something like Against the Giants and would like to get that opportunity for the Fighter to fight through all these Fire Giants to reach the Chieftain in a desperate attempt to save his pals, you have an easy way to do it without handwaving all the dice rolls.
I'm no proponent of 4E, as I see a damn lot of things I dislike in its design, but that particular bit is cool, isn't it?
Also, am I correct that this "minion" setup usually involves great numbers of them?
Since HP are abstracted even more in 4E (with healing surges and all), shouldn't creatures get more HP in a pack, since pack tactics help them avoid injury? To be consistent in the treatment of HP, it doesn't really make sense that More Monsters = Less HP per monster.
Yep. Giving the status of "minion" to a monster basically lowers the threat he represents. He counts as 1/4 or less (don't remember exactly) in the calculation of the "encounter level", if you will (in 3rd ed terms).
There are also "Elite" and "Solo" set-ups like this that modify an original monster to make it tougher. I don't "need" these set-ups, personally, so I don't plan on using them.
Now, I absolutely don't claim that anyone playing C&C will just love the idea. That'd be bologne. It does exactly what I wanted to achieve in regards to the Fighter, for me, by taking on the problem from the outside-in instead of inside-out (which would have been... modifying the Fighter class).