
For those curious: 2H Weapons are meta for Necros partly from doubling the most powerful Aspect, but also because it's exponentially better for the Gem Strength Transfiguration.
We know the Gem Strength multiplier is the chase Transfiguration, but it's also why 2H are so popular. If you have a 1H and Shield or Focus, you can use two gems for up to +32% Multiplicative Damage bonus (one per item), and the Gem Strength Transfig affix maxes out at +125% after Masterworking (or higher if you're crazy lucky and the weapon is still modifiable after Transfig, like mine).
Two Flawless Horadric gems (32% muti) normally provide up to 1.64x DPS if you have no other All Damage or matched elemental damage multipliers on your other gear. Max Gem Strength rolls on both items move this up to 2.44x DPS (2 x +72% multi + base 100%), an overall 49% DPS multiplier. That's almost an offensive Aspect on its own, and it's why Gem Strength is so strong.
However, for a 2H weapon, you still get the two gems, but the gem bonus doubles for a 2H; it probably shouldn't, since affixes that double on 2H usually do so because you only have one item to use, but here you still have both gem slots. Anyways, doubling this now gets you up to 3.24x DPS (2 x +112% multi + base 100%). This is a difference in about 33% DPS, which is like getting half an offensive aspect for free.
Combined with the benefit of doubling your strongest offensive Aspect (often good enough on its own if the Aspect is strong enough), the unusual doubling of the Gem Strength multiplier is the reason 2H weapons rule the roost right now.
(And did I make this into an educational post so I had the excuse to post my crazy lucky weapon roll? Maybe.)