Dual-wielding has been on the brain lately, both with a new guildmate leveling Enhancement as well as my DK acquiring his second spec, a Dual-Wield Frost DPS spec (0/53/18). The contrast between the two dual-wielding classes is pretty striking - the melee Shaman relies heavily on spell damage for DPS these days, and much of Lolknight DPS relies on abilities that scale with physical damage and actually making contact with your target.
When it comes to "not missing skull-level bosses," caster DPS have straightforward goal: reach hit cap, then focus on other stats. For my Draenei Elemental Shaman, this magic number is 342. (source)
For dual-wielders, the hit cap idea is less straightforward. Since there's a huge to-hit penalty imposed on dual-wielding (24% instead of 5%), the actual hit cap for DWers (untalented) is in the 800s (source). Of course, the sheer number of hits-per-minute exceeds those of say a Mage, who's large nukes take several seconds to cast. So at some point, the DW's misses become acceptable, assuming other stats like AP are in good shape.
Thanks to math junkies and spreadsheet jockies (/salute Elitist Jerks), "soft caps" and other hit rating milestones have already been figured out for us. :)
Special Attacks (Yellow Hits)
The first hit milestone relates to special attacks like Stormstrike. This number is very low and attainable almost without trying: 66 for Shaman (33 for Draenei Shaman). (source)
Since Wrath, spell damage has become a much larger part of Enhancement DPS - so much so that even Magma Totem is considered part of the "rotation," even against single targets. The conventional wisdom now seems to be the actual soft cap for Enhancement is the same as the hard cap for Elemental: 368 (or 342 for Draenei). (source) Melee hits will still miss at this number, but as long as your specials and spells are hitting, you should be free to buff your other stats (AP, crit, haste) - any hit rating beyond that number is bonus.
Death Knights were clearly built around the notion of using large two-handed weapons for DPS and Tanking, so for those of us who "just like the idea" of dual-wielding, we have a large hill to climb. Our DW talent is Nerves of Cold Steel, inferior to a Shaman's Dual Wield Specialization or a Rogue's Precision.
After some Googlin', the DWDK hit numbers I've been able to come up with are upwards of 400-500 (source). Ouch. In my own testing, there does seem to be a significant difference as hit rating is added, but my DKs gear is (at this point) barely passable for heroics, with hit rating in the low 300's and AP only in the upper-2000's. I have a feeling that DW scales better with higher gear levels, so we'll see!
Lots of Other Factors
Whether you're a Titan's Gripper, Enhancement Shaman, Death Knight or Rogue, being a successful dual-wielder ultimately comes down to some combination of things: theorycrafting, talent specs, rotations, weapon imbues, and of course, player skill.
Now if I could buy a third spec, my Shaman could 1) have his cake, 2) eat it and 3) throw frosting at Patchwerk with both hands, too. In the meantime, I'll fulfill my dual-wield fetish with my gimped DWDK. :)
After giving it my best effort of getting my DK to almost 400 hit rating with heroic-level gear (mix of blues and epics), I was averaging maybe 1,700 DPS on a single target. By switching over to a more typical two-hander DPS spec, he can now reliably output 2,200+ in the same gear. Looks like if I really want to get my dual wield on, I have to roll a Gnome Rogue. /sigh