The Nitty Gritty of Scaling
Of course, beyond the arguments related to RP and lore are the arguments related to scaling and viability. Ghostcrawler has said that dual wielding and 2-handed weapons should be interchangeable and equal at the end game. Whether this is actually achievable is very much in doubt.
Once you have adequate hit rating, for pure DPS, dual wielding traditionally works better. Dual wielding does more overall white damage. For Death Knights, talents like Necrosis and Blood Caked Blade will scale better with dual wielding. AP scales better with dual wielding. While we do have many weapon-damage related strikes that scale better with 2 handers, there's some question as to whether they will scale well enough to keep up in the end game.
Death Knights are hardly the only class to face such a problem. Most DPS Warriors go for dual wielding Fury these days for min-maxing purposes, with the odd 2-handed Arms Warriors mostly for Blood Frenzy. However, Ghostcrawler has made it clear that the devs want both keep equal, so that an individual Death Knight can choose either for PvP or PvE. But to that, they'll have to make some choices as to what to nerf or buff to obtain that equality.
The choice the devs tried out in the latest beta build is to nerf some of the key talents that make dual wielding scale better. Necrosis was nerfed from 25% extra damage to only 10% extra damage, while Blood Caked Blade now only does 20% weapon damage plus an extra 10% damage for each disease on the target.
Unfortunately, this hasn't actually solved much. Since Necrosis still works off the exact same mechanic as before, it still scales better than 2-handers. As for Blood Caked Blade, well, it's pretty bad all around now, with wide reports of it coming dead last on almost every Death Knight's damage meters in a normal rotation. In other words, it's not even worth the 3 talent points now regardless of what your weapon preference may be.
As far as solutions they have yet to try, the most obvious is probably upping the base weapon damage percentage of certain strikes. Since Death Knights only count main hand attack damage in their weapon damage strikes, 2 handed weapons jump far ahead in calculating those.
The problem is, it's just not far enough ahead. Plague Strike, for example, only does 30% of weapon damage at base. Bumping these instant strikes up would definitely be a boost to 2-handed weapon damage.
But boosting instant weapon strikes brings up a whole new can of worms. There's some concern that boosting them far could make 2handers overpowered as compared to dual wielding, especially at lower levels. Adding more instant weapon strike damage could also push Death Knight burst damage into overpowered territory as well (Not that our burst damage is too amazing right now, according to some). So it's a delicate balancing act on that end as well. Still, instant damage strike boosting may have to come sooner or later to keep 2-handed weapons competitive.
Dual Wield and Variety
As for me, I'm a 2-handed die hard. I don't mind dual wielding per se, and I won't say I won't experiment if two very nice one-handed weapons come my way, but I plan to stay 2-handed for my Death Knight career for both RP and play style reasons, and I wouldn't shed a tear if it disappeared from the Death Knight arsenal. I honestly have to wonder who it is who came up with dual wielding for Death Knights. It's rather shoehorned into the feel of the class from my way of thinking. There's not even a set of dual wield weapons rewarded out of Archerus quests.
Still, the devs seem to want it to stay, and a small but vocal faction of beta testers support it as well. Whether it's worth keeping dual wielding, though, is a question that's probably worth asking. Generally, the biggest argument for dual wielding as that Death Knights need the variety. Of course, the counter argument to this is that Death Knights do, in fact, have plenty of variety. They have 3 talent trees that, in theory, will each have a separate Tank and DPS build.
But there's a lot to be said for more variety too. A lot of Death Knights have been asking not only for a 2-hander only class, but for the ability to keep their original runeblade throughout their career as a Heirloom item due to its looks. That right there gives me pause. My first character was a Feral Druid, and he was my main through the early game, through 40 man raiding, and then through 10 man raiding after Burning Crusade came out. I had to stare at the same Bear model and the same Cat model the whole time. I don't want to repeat that on my Death Knight. From that point of view, maybe the option of duel wielding just for a change of pace is a good thing. Even if I'll never use, it's nice to have options.
Overall, dual wielding is probably here to stay. Here's hoping, though, that they can find the right balance so that I can face down Arthas with my 2-handed runeblade without feeling like a newbie for not dual wielding instead.