It seems to me that D4's itemization is based on which "aspect" (special power in legendary items) you got from legendaries. And since D4 won't have sets (at least not initially), you'll likely have to design your build around which "aspect" you have and what synergies they have.
I think this is probably a more interesting take than D3's set system. Basically in D3 you make your build around the set items so there are not too many variations. D4's system potentially can have much more interesting builds (though I can imagine some might be too powerful and will be nerfed).
Of course this is only the first 25 levels, and I've only played two classes (barb and druid). I played barb last week and basically I didn't know what I was doing, adding to the fact that barb is probably the weakest class now so it felt very underpowered compared to the druid I played this week. But I do believe that class balancing is not the main point of this beta, and considering that Blizzard's history on balancing, I'm not too worry about it (actually I worry about they too focused on balancing).
[EDIT] Probably partly for easier balancing, "aspect" does not stack if you have multiple items with the same "aspect." For example, let's say one aspect is "increase effect X by Y seconds", if you have one item with "increase X by 6 seconds" and another with "increase X by 3 seconds" and you equipped both, only the 6 seconds one will be active, instead of getting 9 seconds. It'll grey out the inactive aspect to show you that's the case (though it's not clearly explained so the first time I saw that I thought it's because the aspect is not for my class).