In world PvP warrior is a fair class, key word is fair. If you have the advantage in gear or skill, you will usualy be able to win. Lacking either of those two and you will start to struggle and fast.
Basically, warrior more or less needs support to get the full power out of it. Something a hunter, mage or rogue simply does themselves. Pair a warrior up with a healer and it is a lot stronger than the sum of its parts.
So in duels against equal skill, against most duellist classes, you will have lower than 50% chance to win. Against certain specs, that can play warriors weaknesses against yourself, you have less than 1% chance to win. Now, at lower levels, you will win more - yet as the ones you duel understand the mechanics better you will soon find yourself rooted and bled to death, being unable to stop it.
World PvP is often short and bloody fights. If you engage, you will most of the time come out ahead. Now, if a rogue engages on you - you already lost. Best case scenario, you can scare it off. Warlocks will be your chosen prey - and they will complain a lot about in the forums. Paladins will be cookies to crack and ferals will leave you dooming. Now, as world PvP often is 3v1 or 1v3, a warrior got plenty of way to keep rolling if you are winning. Most world PvPers will scatter when you land, to try to escape, then you charge in and they decide to fight. When their healer/support dies, they again try to get away. Looping you into feeling like the fox in the henhouse. Feeling that victory rush!
BGs, we are a whole different beast all together. Remember when I mentioned unsupported? Yeah, once you got a healer and support, you will tear people to pieces and they will already have that forum-tab on alt-tab for when you turn their screens grey. Rogue, ferals, mages… don’t matter. With a healer at your back, there is no class or spec that is better than warrior. What small advantages they had, stripped away as their flesh soon the be from their soul.
May Elune guide you 