Hello and welcome back!
I can tell you what I know, you have missed a lot in the past three months!
First off, threads that you need to check up. To keep up with what you missed in the 39 bracket.
The Tournament
Highwav recruiting
Highwav new GM
39 LFM + funding
Ethereum prison key
Queue site
Warlock
Warlock are a support class in the 39 bracket. They are not good in duels or Arena. They are to squishy. Fear and seduction share dimenishing return which makes it harder for you to keep the target off you. But warlocks are very good support class in battleground because they can CC 2 targets at one time. But as I said earlier they can't handle the burst. Their dps is based on dots which is bad because of all the burst in this bracket. They do slow damage over time, and can't put down a healer. (not as we tried). So I wouldn't go a warlock if you intend to play duels and war games.
Protection paladin
They are a strong spec, Paladins is strong overall IMO. But lets keep it about protection. They have the armor of a tank and are near the damage of a dps. The avenger shield (silience) is the one thing that make them so good. It does good damage + silience the target! Which is very effective. They also have stun + lesser damage cool down. With the amount of health you get you will have a strong survival.
Resto druid
Same reason as warlocks but with heals. They have weak casting heals and their dots are not enough to keep up with the burst that is in this bracket. So resto druids are not good as healer alone, but as a support healer they are good. So go resto druid if you want to play in Battle Grounds and be a support class.
Resto shaman
I'm not sure about these classes but they seem to be good. If you want to know more about resto shamans you should talk to
Playroom in Wreckin Balls.
Priest
I have only duelled a priest twice and both times I won as prot paladin. All I did was to survive until the priest would go oom. So I think priest are strong as healers. They have physic scream and mind control, which make them a good cc class. Other than that I don't know.
Remember, these are all my opinion and can be wrong.
I wish you luck and I hope to see you in the Sewer of Dalaran!