A Tier: Hunter, Shaman, Priest and Mage
A Tier classes are still EXTREMELY strong at this level and can absolutely compete with the best classes in the game, especaily if in the right hands. The biggest difference between these and S Tier is that they tend to have a few more unfavorable matchups or weaknesses that good players can easily take advantage of.
Hunter
Hunter are looking REALLY strong at Level 30, and one of the biggest reasons is just how much damage they can put out while keeping their opponents at a distance, and of coure the MASSIVE amount of utility the new pet families bring. 
Marksmanship in particular has some incredible burst damage and 
Beast Mastery benefits massively from how powerful pets have become in WoW Forever. Pets now scale with your stats, which makes them significantly more dangerous than they were in Classic.
Hunters also have some fantastic tools for controlling fights. Between 
Concussive Shot, their traps, and their ability to constantly apply pressure with both ranged attacks and pet damage, while dealing semi decent melee dmg if they too, hey can be extremely difficult to deal with, especially for melee classes. One of the biggest advantages Hunters have is that their pets can continue putting out damage even when the Hunter is being pressured. This is particularly noticeable with Beast Mastery, where pets can be surprisingly difficult to kill while still dealing a TON of damage.
Overall, Hunter is one of the strongest classes in the game right now, with Marksmanship and Beast Mastery being particularly nasty in the current Beta.
Shaman
Shaman is a bit of a weird one right now because 
Enhancementt is an absolute MONSTER when everything lines up, an it's a counter to one of the S Tier classes, 
Warlock, but the class as a whole has some pretty noticeable weaknesses.
The biggest thing going for Enhancement is its INSANE burst damage. Between 
Windfury Weapon and 
Stormstrike, Shamans are capable of absolutely deleting someone if they get a few good procs. Seriously, some of the damage Enhancementt can dish out at Level 30 is wildy ridiculous. They've also got some fantastic utility with 
Purge, their plethora of Totems, 
Ghost Wolf and more. One of the more interesting changes in WoW Forever is the ability to attack while in Ghost Wolf, which gives Enhancement another way to maintain pressure while staying mobile. And unlike a lot of other melee classes, Shaman can also heal themselves, which makes them surprisingly difficult to deal with when they get the opportunity to reset and recover.
However, Enhancement still has some weaknesses. Getting kited or controlled can be a MASSIVE problem, especially against classes that can consistently keep them at a distance. Their damage can also be pretty inconsistent depending on how often Windfury Weapon decides to cooperate. 
Elemental and 
Restoration aren't looking nearly as impressive at the moment either, with Elemental in particular struggling to put out competitive damage compared to Enhancement.
Shaman are absolutely capable of winning duels against classes ranked above them, and I wouldn't be surprised to see Enhancement climb higher as players continue experimenting with builds. But for now, their reliance on burst damage and some difficult matchups are enough to keep them in A Tier.
Priest
Priests rececntly got their 
Penance nerfed, which brings them down from S Tier to A Tier. They are still absolute MONSTERS in Level 30 PvP right now, with 
Discipline in particular being one of the strongest dueling builds in WoW Forever.
The biggest thing going for them right now is just how much they can do while being incredibly difficult to kill. Between 
Power Word: Shield, Penance, 
Psychic Scream, and their ability to dispel every buffs, means they've got an answer to almost everything. And the scary part is that they're not just sitting there healing themselves. 
Mind Blast can dish out massive damage, Penance provides an insane amount of value and damn ear full healing them, but also able to crank the damage when needed as well.
Their survivability, damage, and mana efficiency make them incredibly difficult to deal with, especially for classes that can't maintain enough pressure to break through their shields and healing.
Mage
Frost 
Mage is easily the strongest Mage specialization for Level 30 PvP right now, and it's not even close,
Between 
Frost Novaova, 
Frostbolt, 
Ice Lance and 
Shatter Frost Mages have an INSANE amount of control and can dish out some pretty nasty burst damage while keeping their opponents at a distance, esepcially if they some good RNG and Crit. The addition of Ice Lance in WoW Forever has made 
Frost even more threatening, especially when combined with Shatter and the ability to freeze targets.
On top of that, they've still got 
Polymorph for crowd control and 
Ice Block for when things get a little too spicy. Melee classes in particular can have an absolutely miserable time against a good Frost Mage. If they can't consistently close the gap, they're going to spend most of the duel getting slowed, rooted, and blasted from range.
However, Mage does have some difficult matchups, particularly against Priests, Warlock and certain 
Druid builds since they can't be sapped in form and they shift out of their Frost Nova and sloes. 
Fire and 
Arcane are both very bad at PvP and nowhere near as well as Frost is at the moment, so most of Mage's A Tier placement comes from just how strong Frost alone is.
Engineering, matchup knowledge, and individual player skill can also make a HUGE difference. A lower ranked spec can absolutely beat something above it if the player knows the matchup better.
