Hello, i am not really new to PVP but this is the first time i am approaching it seriously.
It really got me down when i couldn't pass the 1550 rating in 2v2s with class which is considered
to be the easiest class (Combat Rogue) while last season i got to 1800 rating with Hunter not really trying that much
First thing i thought about was the lack of knowledge regarding other classes. As a former Hunter
I know that class inside out and i think that is the reason i find it so easy to beat them.
So i am asking what abilities should i definitely be aware of for every class. I would be really grateful if you could help me.
Especially with defensive cooldown.
Not to give a blanket (and somewhat asshole-ish) answer, but you should be aware of all of them. To have full awareness of what is going on, you should be able to determine each ability that your opponent is using, as well as what abilities to expect so that you can make predictions based on what abilities they have available.
There are so many different abilities and cooldowns for each class and spec so it would take forever to list them all out. It just comes down to practice and experience playing to learn them all. Obviously this is not something that happens over-night...but the more games you play, the more opportunity you have to learn different abilities.
Some types of abilities you should focus on are ones that can directly counter what you are trying to do, but again, you should aim to learn them all. Using combat rogue as your example, abilities that are stun breakers such as icebound fortitude or even the pvp CC break trinket, or defensive cooldowns that can be used while stunned such as barkskin or pain suppression are things you should be keeping close track of, as your end goal is to catch your opponent in a full kidney shot, while having red buff, so that you can killing spree and win.
One of the best ways I found way back in vanilla when learning to PvP was dueling in your spare time. When dueling you will get to experience first hand how different classes react to your abilities, as well as giving you experience on which abilities you might see when fighting that class in other PvP situations.
Another great way now is skirmishes. It gives you a low stress arena situation where you can see how some different abilities work in an arena setting.
If you had specific questions on specific classes that might be easier to give a more direct answer, but there are so many abilities (even after all the pruning) that it would take forever to list them out for every class.
A great way to learn is to watch the gcd tourneys. They have cooldowns listed, and the commentators will let you know when key moves are used. Otherwise, playing the classes that give you trouble will help a lot. But if you don't have that time, you just have to practice a lot.
Consider getting an addon like omnibar to track enemy cd's in arena. It'll be great for tracking key defensives (e.g. displacer beast, ironbark, nature's swiftness), and it is a godsend if you ever play a caster. And get a trinket/DR tracker, such as sArena with diminishing returns or Gladius. Keeping track of someone's trinket is pretty much mandatory for when you want to get a kill. A lot of your killing sprees will be spent getting defensive cooldowns, and trinket is usually the most important. After that, you can wait for your next red buff, put them in a full kidney, bomb if necessary, and deliver the damage.
As rorian said, listing ALL the defensives and abilities to keep track of is pretty extensive, and it would be hard to keep track of even if you saw them. If you have any particular class in mind that is giving you trouble, it might be easier for us to give advice on what to watch.
Also, don't feel bad if you can't climb too high. It just takes practice, and climbing is a lot more difficult in the current state of the game than it was in previous seasons (due to some changes in MMR and points gained), especially if you don't have a full grasp of the each classes' abilities. It is a lot harder to call targets unless you know how viable each target is. If you are only concerned with 2's, I would recommend mage rogue or hunter rogue since I find those the funnest. Healer/rogue obviously works, but it isn't as enjoyable imo.
All Major defensive and offensive burst cooldowns of every class/spec and their crowd control effects.
You should know what Cloak of shadows does, what it looks like and how long it lasts,+ Evasion, vanish, ShadowDance,Killing Spree, Vendetta, Shadow Reflection, Blind, sap,gouge,Kidney Shot, cheap shot, garrote.