3.x monsters are built the same as 3.x PCs, sort of. Each monster type is a sort of "class," with a HD type, a BAB progression, saves and so forth. The very existence of these "classes" subtly suggests that they exist to maintain an in-game reality. The beauty of all these design hoops is that gamers -- both DMs and players -- who know about these "monster classes" tend to create their own curtains to rationalize them all on their own. "Dragons have a d12 HD because their bones are super-dense," for example. Not everyone is so specific in their rationalizations, but many of us tend to just assume there're good reasons for this stuff. Myself included.
Barbarians don't have d12 HD because they're dense; neither to dragons. But they do have it for a reason. Perhaps because of physical superiority, or perhaps because of luck/skill/will/etc.; this goes to the vagueness of hit poitns. But there is a reason, in both cases. The reason is not "because that's balanced" or because it fills a mechanical niche, but something relating to the in-game reality.
4e drew the curtain back, and drew attention to the hardest part of monster design: challenging the players and PCs. "Look," 4e says, "all those monster classes and strange little rules aren't about immersion or believability. They're really there to help you challenge your players without overwhelming them. And
Actually, 3e drew the curtain back. It said that what really matters about a monster is what it is relative to the reality of the world, not relative to one group of player characters. What really matters is how strong it is, how smart it is, how magical it is, not how balanced of an encounter it is.
Problem for some gamers is, calling out what's actually important in balanced monster design ruins the illusion that all those "monster classes" and quirky rules are important.
The problem for others is that this isn't an illusion. If my monsters weren't designed by the same rules as their characters, they'd know it, and they'd call B.S. on it. And they'd be right. What is the point playing two different games at once, one for the DM and one for the players? If everyone's not playing by the same rules, what's the point of having rules at all? If the rules aren't there to describe reality and make the outcomes fait, why not play without them?
Near the end of my 3.x days, I started writing monsters beginning with their CR, and then basing other stats on that.
I stopped using CR because I realized that the challenge a monster posed to the PC was only peripherally relevant to how I was going to use it. And XP, for that matter. My encounters are not even close to balanced.
Anyway, yeah, I have been specifically told by detractors of 4e that drawing the curtain to reveal the nuts and bolts of the game is one of the things that 4e does wrong. Because seeing thru the illusion ruins the magic for them.
That's one perspective. Here's another:
The ambiguity and diversity of older editions' rules let people cultivate a variety of philosophies. 4e didn't reveal an approach to monster design that was always there, it picked one out of the many that were always there, and effectively excluded the others. For those that enjoyed this approach (i.e. monsters as cardboard cutouts designed to challenge PCs for less than a minute and then die), this is okay. For those who had a different idea, the response is: "What? This is all an rpg can be?"
It's sort of like watching The Matrix, getting a thrill as Neo shoots into the sky and an uncertain future, and then watching the sequels. Really, that's what they were thinking? The version of what happens next in my imagination (my illusion, if you will) was
so much better. A world of possibilities becomes one really lame reality.
I used to think of monsters as being just challenges to the PCs, but now I use them as characters as part of a story, in part because of the 3.5 monster/character design rules. I can't imagine going back/having that taken away.