Every single thing is limited to you, to the way you think about things - just as in the real life, you might find a piece of paper useless, but there are some people who could use that to kill someone. As a fact, there are some features in World of Warcraft that I call "game mechanics" what create limits to things, involving classes and their relationship to each other, but I pretend that there isn't any of these mechanics that ruins the PvP for any class. Everything depends on you, there for if you are not satisfied with any kind of performance you provide ingame, search the reasons in yourself.
Practice, Think, Learn. It's true of anything; if you pay attention to something, if you try to use your mind, if you try to understand it you'll have far better results than if you would do it brainless, without considering the circumstances. If you are about to maximize your performance-possibilities, it's not even close to enough if you play twenty-four hours a day, seven days a week, because it will only pay itself if you think about the thing you do. Simple as that, you must know the reasons of every thing happens to you while you do PvP, if you invest the time and energy in summarizing, you have the chance to learn from it, and be better, find a way you can solve the problems you meet.
Some people are not capable of it, they are playing the game since beta, they are managed to get PvP Rank14 and they still lose to most opponents, and without the right attitude and self-criticism, they start to look for the excuses and problems in the other classes and the game. What I'm pointing out: you must force yourself (if it doesn't go by itself) to think over everything. Pay attention to all the fights you play in, to what your enemies do, when, why.
That's the key, and If you get that, the next step is the time. The more you invest, the better results you get. Be objective.
People use this word (balance) many times on forums, most of these times they claim that something is not matching that word. Also, a lot of players like to talk around about the World of Warcraft's "Rock-Paper-Scissor" mechanism, like "Mage beats Warrior, Warrior beats Rogue, Rogue beats Mage". I hardly pretend that both of these are false.
It's not even possible to ask for some kind of a perfect harmony between the classes, hopefully I don't have to point the reasons. It been a very long time since something crossed the line so much that it made some aspects of the game unenjoyable. Today we have more matterful factors that classify the balance (like we have more abilities, new stat system, the Resilience, and so on) of the game, and in my view there's nothing that would need a radical change, otherwise it will continue ruining the game. Naturally there are some things that would require little changes. I've seen many good ideas that would bring some new colors to the gameplay (or into the Warrior class), some that would fix the current issues, and probably a few of them will have a place in the game in the future.
The whole thing is truly subjective, because even if I say that I do have a chance against any kind of enemies, you might say that I haven't faced opponents that were good enough - and to continue the line, I could ask something similar from you when you say you can't win against anyone. But just because there's someone who is satisfied with something, but the other 99.99% of the people not, it's not wise to make the decisions according to the first one. And that's what makes World Of Warcraft a great game in my eye, like all the time, with minor problematics, Blizzard could manage to keep things at the right side of the edge, everyone has their own chance to find the levels they enjoy the game on. What causing the problems then?
Many countless people don't have any idea about what level they play on. You can name these levels by any details of the game and the players; how well they can play ; how good their gear is ; talent specializations; dedication ; and the list goes on. Now, if you are playing PvP casually, you have only average gear for it, why would you want to win against someone that has the full gear, and far more experience than you in PvP? There are numberless opponents that would match your criteria better, therefor if you don't put the maximal effort in the thing you do, don't expect sucess without problems.
Why all of these things are important? Because apart from subjectivity, we do stand a chance against any kind of opponents. Other opinions are based on skill-difference, gear-difference, and other things that builds PvP on the surface. Also, don't forget there are many differences between an average and average player, between good and good, also between the insane and insane. And that's the reason why the "Rock-Paper-Scissor" fails in my mind, there are some classes that have minor (or not only minor) advantages against another, but the way these two players play the game is far more important in most cases. Basically we can stand proudly in every Arena team-bracket, though we have some limitations to what our partner has to be.
So what do we have to do to be viable in solo situations? What do we have to do to be useful in team fights? It's very simple, and insanely complicated at the same time - just as with other classes.
The fact is, you can learn to play quite fast, but that's only enough to have some fun for the same level you play on. But if you provide some dedication, and start to work on getting better, you'll see that we do have abilities that can be used in wrong and good ways, even if the difference is hardly noticable, it's there, and it's important.