TWF just got more realistic. (it was also a pain in the ass to calculate the penalties depending on what feats you had, way too complex, hopefully fixed)
Being able to wield two equal sized weapons was ridiculous. I do like the default two weapon style is an either/or attack. It will be nice to see what feats/powers are generally available to augment this fighting style.
Maybe you could designate the off-hand to do something useful through a feat (AC, damage or to-hit). Or you roll the second weapons damage on a a critical. Simulating a two weapon strike.
I hated the fact that all weapons were very similar or crazy in 3e. Weapons should encourage a differentiation of fighting style and, from this preview, I think 4e is well on the way to doing this.
EDIT: What the hell are they going to do with Drizzt? I suppose exception based design. Feat: Drow-Bad-Ass-Figthing Style...