PvP in WAR - particularly group PvP, like in Scenarios or taking a team into the RvR areas - is a blast, and the game is clearly built around it. Players earn both experience and money for slaying enemy characters (and can even get items - though you won't ever lose anything by being defeated in PvP). Since you can queue for Scenarios at any time from anywhere on the map, it is entirely possible to level completely through PvP without ever killing a computer-controlled enemy.
The "Bolster" system is a particularly nice touch that means lower-level characters can actually contribute to the fight instead of being dead weight. As soon as a character enters an RvR area (be it in a Scenario or out in the world), they will have their stats and power boosted to what they would be at level X8 (a level 3 will become a level 8, a level 15 will become a level 18, and so on). They won't have the gear or the abilities that a natural level X8 character would have, so there's still incentive to level up, but it means that players can contribute to the war no matter the level.
In addition to experience, victories in PvP will accrue "renown" for your character, gradually filling a sort of secondary experience bar. As you advance in renown ranks, you can buy armor from NPC vendors, so a player electing to advance solely through PvP can still reliably upgrade their armor (that other players might get through quests).
For the first three tiers, controlling a zone is nice but hardly essential. When your Realm controls a particular zone, characters of that faction get slight boosts to experience gained, money earned, and so on. At the fourth and final tier, however, RvR explodes in importance for everybody. Capturing a zone will allow your faction to contest more and more of your opponents' territory, with the ultimate goal being to assault the enemy capital. City raids are the ultimate goal of WAR, yielding the best loot and the most exciting combat.
While every race was originally intended to have their own capital city to protect, four of the six were infamously cut not too long ago, leaving only Altdorf of the Empire and The Inevitable City of Chaos. It is worth mentioning that both Altdorf and The Inevitable City are gigantic and truly impressive bits of game design, but one can't help but be disappointed that the other four won't be in for a good while - if ever.
WAR manages to avoid one of the usual pitfalls of the MMO genre, making new characters feel too insignificant. There are no beginning quests to kill rats or snakes or other small critters - instead, from the very outset, your character is thrust into war. "Welcome to the fight, these are your ancient ancestral enemies - go kill them." Even pieces of low-level equipment look less like random rags and more like cool armor and clothing (if not particularly decorative).
The downside to this, though, is that ... well, your enemy never really changes. A level 5 Greenskin will have fun slaughtering Dwarfs on the field of battle. A level 35 Greenskin will still be slaughtering Dwarfs. When combined with the fact that there's very little noticeable difference between different pieces of armor and weaponry you acquire as you play, it can feel like you've made very little progression with your character after hours of play. Thankfully, It's easy to travel between the different battlefields, though, so a Black Orc tired of "bashin' stunties" can go spend time killing "da pointy-earz" instead.