The monk stops the attacker by being the first person in line.
When fighting in a tunnel, sure he does. Unless the attacker uses Bull Rushing, at any rate.
Unless the DM controls only monsters that have pre-obtained knowledge of which player is most dangerous
Monsters that would be in no way able to use their brains to determine who the most dangerous party members were, which ones were the weak casters, or make any sort of tactical assessment other than a blind charge at the first person they see. Seriously, I'm sure anything with an Int of about 8 or greater will notice that people who can rewrite reality on a whim are pretty high up there on the danger meter.
they should attack what looks like the easiest prey.
Which would not be the squishy wizard?
The unarmored creep carelessly walking ahead should be the first to get ambushed.
d4 hit dice, no muscles, sissy clothing at the back vs someone who obviously has mucles, ie, a
strength score of 14. Who is going to get ambushed again?
The monster shouldn't know who to attack until the wizard pops off her first spell, at which point the battle might be half over.
Unless they are mentally incapable of noticing that someone's wearing either a mage's robes. or a bondage outfit if you're Hennet, and concluding that that person may be a caster of some sort.
but moving out of combat with the monk means getting more damage from AoO (albeit not as great as the wizard's damage, but becomes critically significant when low on hit points)
Yes, a whopping 2d6+2 damage with an attack bonus of +11 for the AoO. Truely an intimidating penalty for a melee opponent that will likely have HP in the triple digits.
And depending what that first spell might be, it becomes more futile to attack the wizard than the monk.
Ah, but that's doubleplus wrongbad metagame knowledge there! Arent monsters supposed to avoid that?
And occupying a 5 ft square is more advantageous than you make it sound.
Not when that's
all you do, it isn't.
The position of pawns on the board has more significance than the position of queens. A well-placed monk stops a monster from charging the wizard or flanking the cleric.
That's not broken, that's
lackluster at best.
I think the problem is that you and I have different opinions on the value of defense.
I think the problem is that the monk's defensive capabilities suck.