Currently half way through Act 1 nightmare with my Monk and reading this thread:
http://us.battle.net/d3/en/forum/topic/5149150706?page=1
is starting to worry me a little.
Anyone having the same issues in later difficulties with the Monk?
Does the Monk really get outclassed by the DH and WD as you lvl up?
The monk is perfectly fine up through act 1 of inferno. At that point, things get a bit dicey for both the monk and the barbarian. If you're really embarrassingly well geared, you can handle act 2, but even the best geared barbs and monks can't really handle act 3 and certainly not act 4 while solo.
They've also pigeonholed you into what builds are effective in inferno because of how ridiculously hard the enemies hit. You have to build tanky past a certain point. You'll typically run a single spirit generator and then heals, blinds, invulnerabilities and other utility skills. You don't get to run a build focused primarily on DPS because melee classes, by their very nature, will get hit, and getting hit without a super tanky build will rock your face on inferno.
I would say that the monk is in a better position than the barbarian. The barb depends almost entirely on a few 2 minute cooldown abilities that greatly increase your damage output for 15 seconds to kill champion and elite packs, which is an unfun concept in Diablo in my opinion. Their survivability, even when geared to the point of having net damage reduction greater than 95% against all damage types, is predicated entirely on one ability: revenge.
Revenge is the greatest design flaw with the barbarian class because it's the only viable way for them, ostensibly meat shields, to recover health consistently. Life pools get so big and life on hit and life leech is so diminished and weak compared to your health pool that the barbarian's few abilities that restore health based on your maximum health are absolutely required in every single build.
Monks have a buttload of dodge, and their tankiness is a little more RNG, but a little less fire and forget cooldowns and spamming a really big AoE self heal like revenge. If you're curious about how the class can perform in inferno, watch this guy:
http://www.twitch.tv/cadburryftw
He's in act 2 soloing as a monk and performing OK. He streams pretty consistently in the mornings on his monk.