The level cap represents the ultimate goal and culmination of the leveling process, with max-level characters having gained access to all class abilities and talents, and being able to access end-game content and equip the most powerful gear. By providing an equal limit for the progression of all characters, the level cap allows for competitive and balanced gameplay in both PvE and PvP.
Note that character level is limited by which expansions the player has purchased, with only players who have purchased the latest expansion able to reach the current level cap. As of May 2016, players with the standard game are able to reach level 100. The free Starter Edition of the game has a level cap of 20.
The game is not over at the cap
When the level cap is reached, players who are used to the leveling game sometimes have difficulty working out how to continue their adventures. After reaching the cap, however, is where the game developers are hardest at work for the players.
Once characters reach the level cap, gameplay turns from leveling to the end-game. This represents the culmination of the leveling process and the beginning of the deeper challenges awaiting max-level characters. While play prior to this point is oriented around accumulating experience and gaining new skills, the end-game represents the polished 'final destination' for players, at which characters are fully developed and ready to tackle the biggest challenges in the game, or to relax and make their own adventure.
For a guide, see End-game.
Players at max level are at the pinnacle of the leveling tree, with a strong advantage over those still struggling below. Zones that were once challenging are now easy; even group content from earlier levels can now be soloed at will. Combat with characters below max level is very likely to end in the max level player's favour.
The dark side of this disparity is griefing; max level players are able to easily disrupt questing areas in use by players of the opposite faction, and can one-shot most enemy characters. Sadly, even guards in most areas are unable to stand against well-equipped max level players, especially as the current expansion progresses and gear steadily increases in power. Of course, the best answer to this is usually to recruit some max level players of your own faction, or switch to a max level alt, and make the griefer regret the day they flew into Goldshire.
There are various upsides to this imbalance. Max level players are able to complete lower level content with ease, allowing them to swiftly complete missed quests and achievements in earlier zones, and can travel through zones without being constantly attacked, since mobs will only attack players several levels higher than them if walked directly into. Max level also makes it possible to solo earlier group content such as dungeons and even raids, making farming loot such as mounts far easier, and for those interested in a challenge opening up a new subtype of play at slightly higher levels.
Max level players are also in a good position to help lower level players, through assistance against griefers, providing buffs, items or gold, or helping them with group quests or difficulties. Max level players often help guildmates level their characters, and may also be able to help with transport if they have a two-person mount.
World of Warcraft originally had a level cap of level 60, but as expansions are released, the level cap is increased by 5 or commonly 10.