1. 4e has rules for what AC a monster can have, there are guidelines in the DMG for ranges of AC based on monster role and level.
I agree. Luckily the guidelines are ones that create monsters that are "fair" fights. They purposefully "prevent" DMs from coming up with the arbitrary 50 AC monster because they either decided to make it up or found a template that "legally" gave it to a creature.
2 3e does not. It has rules on what things add to AC and in what ways, but no rules on how much AC to give a monster. There are no limits on how much natural armor a monster can have for example, only examples of what some exemplar monsters do have in the MM.
See above. Precisely right.
3 Majoru, your example is not that you did not play by the rules and therefore did not play fair, your players are complaining that you played by the 3e rules legally and they don't like it. They want you to stick to core MM stat blocks apparently. You played "fair" by your example of what you say your players required in 3e. 3e rules allow a ton of arbitrary flexibility while still playing by the rules.
Players don't like being hosed over. *I* don't like being hosed over, so I'm with them. The game is no fun when you need 19s to hit an enemy. The battle drags out impossibly long and it likely ends up in you dying. Losing sucks. Losing because you had a bunch of bad die rolls or you made a really bad tactical decision at least feels like a fair loss. It was a loss that was preventable with some better luck at some slightly more intelligent play. Maybe it was because you only a 14 into your strength and the next character you make up with have 18 because you learned.
But if you come across some custom monster that manages to have 10 more AC than every other monster anywhere near its CR...well, it feels like it isn't fair anymore. Of course you're going to lose....even if it IS legal. I spent the better part of the last year of Living Greyhawk(since I was a Triad member, helping to run the campaign) reading mailing lists filled with people complaining about how authors had used what players considered "cheap" but legal techniques to build monsters. A lot of authors retaliated, saying that if there weren't a bunch of players out there using cheap powergaming tactics for their characters, they wouldn't have to write so many nasty encounters...and so on.
NEw monster at AC 36. 10 base, +8 full plate, +2 shield, +16 natural. Done. Legal according to the rules of 3e. And they will only get non magical full plate and a shield as loot.
Yeah, I know this is legal. I know that it was fairly easy to add 10 points to someone's AC simply by adding full plate and a shield. It was a favorite tactic of nasty authors everywhere. After all, nothing in the rules said to raise the CR of an enemy simply because it was wearing armor....even if the point of CR was to evaluate how difficult it was to defeat something and armor made the creature significantly harder to defeat.
It's a risk/reward thing. CR determines XP for defeating a monster. It also, according to the rules determined how many of a creature you could use against a party. A number of players felt that if a monster was suddenly twice as hard to defeat, they should get twice as much XP for it. But the system didn't tell you to increase CR. So, you didn't get any more.
Your players memorize monster stats but apparently ignore the parts about adding levels, advancing by HD, adding templates, and the ability of the DM to create their own monsters.
They know about all that stuff. Custom monsters were considered by everyone in our home group to be super cheap and not a tactic that should be allowed. Mostly by equal agreement of DM and players. Way too easy to unbalance the game that way.
As for the rest of it. Most of my players had memorized the effects of increasing hit dice, adding class levels, adding templates and so on. If you ran into an Orc, there should be some visual clue that it wasn't a normal Orc. If it has metal skin, it might be half-iron golem. Scales? Half-Dragon...and so on. The only thing that didn't have a visual clue tended to be Class Levels. Which is why one of the dirty tricks of LG Authors was to add one level of Warrior to enemies. Since it was an NPC class, it didn't add anything to CR. But it could give a bonus feat and some extra hitpoints to any monster in the game, legally.
My players knew what the general range that an AC could be given the options in the book. Given our "no custom monster" rule, there was virtually no way to increase natural armor of a creature. You were forced to scour books for creatures with the best starting natural armor and then modify them with hitdice, levels, and templates to make them better. If a DM came up with an awesome combination of legal stuff from the rules....players would accept it. But probably still get annoyed if you went "overboard".