Brewmaster stat priority is still being theorycrafted. it's heated because different stats are good for different fights.
Monks are a tricky tank. ALL of thier surviability is active. they get a small health boost, and no armor boost. stagger takes some of the spikeyness from having only leather armor, but the damage reduced is dealt back over time. they get a small, passive boost (5%) to parry. the standard agility => dodge conversion, and that's that. Leather dosen't typically come with tanking stats, nor are tanking 2h weapons very common. (even tanking 1h's are pretty uncommon this time around, and they always come with str, so it's moot anyway.)
Expertise cap is very important. A missed attack = no chi = no mitigation. The same is also true when the monk is stunned, no chi incoming, no defenses (and they still don't have that extra set of armor that, say, a death knight or druid tank gets for being in tank mode.
It isn't impossible to tank with them, and it shouldn't hinder your progression if he's good at the active mitigation model. But going from paladins, who were one of the more 'passive' tanks in cata to brewmasters is kind of a night/day comparison.
Part of the problem is that a lot of people are still arguing over the optimal stat priority. Some people say "Crit isn't as good as parry" some people put more value into mastery. The reality, (in my eyes) is that the gear priority for monks is HIGHLY dependant on fight. For magic fights, dodge and parry do nothing for monks, and mastery is chief. for melee fights mastery is a flat % reduction, where dodge + parry = 0 damage. Haste = more energy (and therefore more chi) and crit gives more dodge through more stacks of elusive brew. (where a slower weapon auto attack crit will generate up to 3 stacks, and dual-wielding crits will be as low as one stack.)
tl;dr:
Hit/Expertise/haste = more chi = less incoming damage. Most avoidance is attack based. Read up on my links below. Ensuring shuffle (which gives +20% parry) uptime is dependant on your ability to land blackout kick. (which also restores health when used from the front). Jab, Tiger Strike, Keg Smash are all dependant on melee hit. I believe Expel harm needs spell hit (which is 7.5% hit + 7.5% expertise, and heals the brewmaster and gives chi regardless of it dealing damage)
Crit/Dodge/Parry = good for melee fights. works the same as other classes.
Mastery = good for magic fights. (see
http://www.wowdb.com/spells/115069-s...-the-sturdy-ox and
http://www.wowdb.com/spells/117906-m...lusive-brawler ) their mastery spreads out a portion of incoming damage over time, putting it on a debuff.
Haste increases energy regeneration.
Crit gives stacks of elusive brew (which is a 9 sec cd ability that consumes said stacks to give the brewmaster 30% dodge, lasting 1sec per stack consumed.)
A high crit means a high elusive brew uptime (each WHITE crit gives stacks dependant on weapon speed, slower weapons generate more stacks). High haste means more chi to spend on mitigation. chi generation is almost wholly dependant on energy regen(haste) and ability to land chi-generating attacks(hit/expertise). Mastery increases the amount of damage per hit moved to the pool of stagger. Stamina increases health, and therefore increases your capacity to stagger damage. Agility increases damage and dodge. Parry is weighted before dodge because it has lower diminishing returns. Your priority should change from fight to fight, which means a good monk is going to have several tanking sets, and will probably step on the toes of leather DPS to get those tanking sets.