Armor set pieces should be weaker then their Unique counterpart individually. However when you get the bonus from having multiple pieces of that set, they should equal the unique items.
This will allow for diversity in item choices. Uniques and Rares should be very powerful by themselves but sets should have the ability to be as powerful except in a different kind of way by having to wear the multiple pieces.
I also think there should be some Sets that scale with level. One thing I really hated in D2 was that you would find a shitty low level set piece when you are level 60+. I haven't payed D2 in so long, but that's how I remember it being.
I don't think they are going to go with the full plate armor look. I could see it showing up as a specific look to a Set, if sets are even in the game, but I do not see it being common. Personally I have no problem with it, but I just think Blizzard is going to stray away from that full on tank feel and give armor a more kick ass feel as in armor that seems more worth for attacking than tanking.