81s and 83 can do well, depending on class and spec. I almost went 83, but decided to stay 80 for better sustained damage mitigation. You'll really want to check available gear, talents and Tier bonuses to see if going above 80 is worth your time.
Edit: The major drawback to going above 80 comes from secondary stat degradation (crit, hit, haste, resil, mastery, etc.)
81 = 24% reduction
82 = 42% reduction
83 = 56% reduction
84 = 66% reduction
For example, the 66% reduction at 84 means you need three times as much of a secondary stat to achieve the same percentage as if you were 80. Or put another way, 200 resilience at 84 = 600 resilience at 80. Even going to just 81 causes an almost one-quarter drop in your secondary stats. So the question becomes, do your gear upgrades, spells, talents, and other factors make it worth losing that much in secondary stats?
84 is deffo worth then 83, but even so ive seen 84 prot palas do 20k avenger on my 80 pala with 600 ress. And that thing takes FOREVER to kill if you are a melee.
I did some research into 81 plate gear, and it dose not look too good. You cant get full 316 gear setup at 81, so you will have to use some of the same gear as lvl 80 use, but you get a much better 2h weapon and incredible JC trinket. Warrior gains Colossus Smash that's your ticket to one shooting stuff. Paladin gets Inquisition, that's another ability that makes your stuff go BOOM with 30% more dmg. DK gets Outbreak, mleh nothing groundbreaking. So that the sum up of plate wearing 81s.
All i can say about 83 is that Frost DK is deffo very viable, because you have a killing machine proc. Killing machine dose not care about stat scaling, when it procs you crit, very simple, So it kind of mitigates low crit chance. As already stated above tanks are also viable due to increased HP pool and innate abilities of prot classes to shrug off incoming DMG.
Hunters? I never seen any proper 80+ hunters but, a 80 full cata gear marksman hunter is going to kill you so fast, that you will have to sh@t your pants once your dead coz there's no time for it when you are alive. So i guess this class is in the same basket as a mage and healers, rly good at 80 no reason to go higher.