I've been mulling this question over for the past few weeks as beta information has been revealed, and almost convinced myself that contrary to the popular build advice of today, most monks would be better off as Dex based in the expansion. Before I take my final TR however, I'd like some other input. It's not one big change that is leading me to this believe, more the convergence of several influences that taken together, make Dex the superior choice. I'll run through them now:
Str is preferred now because it does not cost a feat to use and it does more damage per hit. Str also has more bonuses available than Dex does, making it possible to attain a higher AB. I believe these factors will become less relevant in the expansion.
For a start, Madstone Boots are one of the primary reasons to be Str based. However, as more gear comes out, Madstone Boots will face increasing competition from items that offer things it does not. As we get more abilities from our Epic Destinies and Twists of Fate, we will increasingly find even as a melee that many abilities we could want are prohibited to use under Madstone Rage. I believe Madstone Boots will be replaced by level 25, taking -4 Str off the maximum attainable in raids.
We are also likely to be remaining in Air Stance. For a Monk that wants max DPS and to not pull aggro, it is the ideal choice. The +4 Dex in this stance has always lent weight to the argument to be Dex based, and it will not be changing.
We will be stretched even further in stats by being required to have 23 base wisdom to access one of our new epic feats. Even though the requirement is strict, the benefit the feat provides seems to be worth it. For Str builds however, the extra levelup points put into Wis instead of Str are far more costly than a Dex build. A Str build is required to have at least 18 Dex for Grandmaster Air, but there is no such requirement on a Dex build to maintain 18 base Str. We could drop to 16 Str and still perform well, placing those points into Wis and lowering the impact to our primary AB stat.
Our Monk flavoured Epic Destiny has no Str increases. Neither does Shadowdancer. The other melee destinies that do offer Str increases are littered with abilities that do not affect/are not suited for unarmed combat. It seems the only way to increase our AB in line with other melee whilst also picking a suitable destiny for our class is to be Dex based.
Shadowdancer is a particularly powerful Epic Destiny to a Dex based monk. Though the early choices in the tree are a little weak, the destiny is extremely strong due to the power of its passives. +6d6 sneak, passive weapon finesse and passive bonus to damage from Dex instead of Str. Why not Grandmaster of Flowers however? It's a fine choice, but for what appears to me to be an entirely different role. A GoF would be better at AoE CC/Insta killing, but weaker in DPS. The GoF abilities also compete with our current monk abilities instead of enhancing them. Ki is a limited resource, and the costs of the new abilities are high enough that no Dark Monk could use them whilst also maintaining frequent use of Stunning Fist, Touch of Death and Elemental Strikes. The 6 second future punch sounds like reasonable damage until you realise that against bosses, it's just a ToD on a longer timer for the same Ki that probably won't proc on damage on offhand/doublestrike. The Ki blasts interupt the attack sequence and actually lower DPS. There are issues with aiming the Ki blasts in melee combat. There are no such problems with 6d6 sneak. It's simple and powerful, and does not compete for exclusivity with any abilities we have currently.
One of the benefits that Dex is supposed to provide now is worthless to most of us. AC. This is changing in the expansion. The range at which AC begins to be useful has been drastically decreased. A Dex Monk will have a significant survival advantage over a Str monk even without a focus in AC gear.
After the expansion, we will see an update to the enhancement system. This will bring a whole new set of changes, and the details we have currently are a lot more 'up in the air' than what we know for the expansion. It seems however that for a monk focused on DPS, the availability of the Assassin as a racial prestige is likely to be of interest. Even without the Int for the Assassin specials, the bonus to sneak attacks and the bonus to hit while sneaking is tempting. This prestige is however...tied to Halfling, which almost always means: Dex build. Halflings offer +1 ab from racial dexterity for a traditional halfelf/human monk build, and another +1 from halfling racial enhancements. The size modifier is another +1 ab. The halfling sneak attack enhancements are another +4 ab while sneaking - enhancements that many suspect will go down in cost in the update. Taken altogether, a Halfing Dex Assassin Monk will have a 9 ab advantage over the traditional half-elf monk of today. You'll see people complain that 9ab doesn't mean as much as it used to, and they're right. It used to mean A LOT. But just because its efficacy has been reduced does not mean that it's not still the best thing you can do to improve your DPS. And that's not just a matter for DPS, a missed attack is certainly a loss in DPS, but even more so if that attack was attached to a tactical attack. It could be a missed Sunder, losing your stack, or a missed Stun, leading to a dead party member. Increasing your chance to hit is always the best thing you can do.
Am I wrong? Am I missing something important that shifts the balance back to Str in the expansion, or should I prepare myself to return to Dex?