Let's see... problems with the Monk:
> Low attack bonus means a lot of missing.
> Can't penetrate DR well with primary ability.
> Can't do much against any monster immune to bludgeoning or that regenerates.
> Can't gain Reach.
> Can't really use all those neat armor and weapon special enchantments.
Requires four stats to be useful.
> Enhancement bonuses add to attack bonus, damage rolls, sundering, and bypassing DR. Ki Strike only bypasses DR. So, those people using magical weapons instead of fists have an even higher attack bonus, and can do just as much damage.
Level 20 Fighter with a +5 greatsword: +25(+STR) to hit, 12+(1.5*STR) damage, plus another two for Weapon Specialization, and a 19-20 crit range.
Level 20 Monk fists: +15(+STR) to hit, 10.5(+STR) damage. 20 crit range.
Now, some people say "carry a Kama/Nunchuk/Siangham". It's just not worth it most of the time. If you get to the point where you use those, you're suddenly inferior to every other melee class (except the Bard!).
> Gets a bunch of near-useless abilities. Not aging? Meh.
> No flexibility in class abilities; every Monk 15 is the same, fundamentally, as every other Monk 15.
> Thanks to the multiclassing limitation it's hard for them to join PrCs.
> Their speed is nice, but after a certain point (50', IMO) it just stops being a big deal. With the ranges most fights are at, you reach a point where you point at the map, say "I move there", and you're done.
IMC we already rebuilt the Monk class. It really needed it, and the version we came up with is something people actually are willing to play again.