Tier 20 is an Updated Tier 6

There are two totally understandable reactions you might have to the following: On one hand, the tier sets we're getting in the Tomb of Sargeras raid are rehashes - they're updates of existing tier sets, with the obvious exceptions for classes that didn't exist at the time - and one could be disappointed that we're not seeing totally new designs.

On the other hand, tier 6 has got to be one of the most popular series of armor sets they've ever made. While I find myself partial to tier 8 (Ulduar) for a lot of classes, Tier 6 is also a set of classics. For example, while I tend to go around in Tier 8 Aegis on my Paladin, I don't think you could argue that there's any set that embodies what it means to be a Paladin than Tier 6 Lightbringer (Yes, even Tier 2 Judgment.)

There's another benefit to this decision, which is that for those of us who like using these sets for Transmog, we've definitely come to a point where their age is showing. Tier 6 came out in 2007, which you'll note is ten years ago (holy crap I'm old.) And as much as I love my Lightbringer set, it just doesn't look like it fits in the same world as all the high-rez models you see now.

You can look at the sets at MMO-Champion.

In addition to bulking up the resolution and doing some minor redesign to be more in line with what we expect these days, there are also color variations for different raid difficulties (of course, at the time of Hyjal and Black Temple, there was only one raid difficulty.) It also looks like the Mythic raid models will be beefed up in the way they've been since Tier 17.

Now, obviously, Death Knights, Monks, and Demon Hunters didn't have tier sets for those raids, what with the fact that they didn't exist yet (in fact, Demon Hunters would have been fighting for the "bad guys" in Black Temple.) So these sets are going to be totally new.

Death Knights get an appropriately skull-covered set with a helmet that looks more like a traditional knight visor than you tend to see in WoW (it's kind of amazing to me that there hasn't ever been a plate set, even at low levels, that actually looks like a traditional suit of armor.)

Monks get a fairly White Tiger-themed set (though it comes in a bunch of different color variations.) It's got your standard giant beads and a strong Xuen theme.

Demon Hunters get a set that looks pretty similar to what you'd expect - lots of demonic horns and such with bright green accents over quasi-metallic pads.

Given that Nighthold literally just came out a couple days ago, we've got a very long time before any of this becomes available, but I'm actually pretty happy about this tier set idea. The question is how hard it will be to get the different color variations.