Gating is the act of restricting access until conditions are met. Many games work by using gating (WoW uses level as a gate, Mario 64 uses stars, Spider-Man: Shattered Dimensions uses level completion), and its often an accepted form of game design. Many people would never blink an eye-lash if you said you had to beat Level 1 of Ghosts and Goblins before moving on to Level 2.
Even Mega Man, which gives you a choice of the first 8 levels, requires the completion of those levels before you head out towards Wily's Castle.
But, what is it that these gates reward the player with? More game.
Playing level one lets you play level 2. Level 2 is harder than level 1, and you don't know level 2. So there's a bit of discovery and exploration involved in level 2. Maybe, level 2 is different from level 1. Maybe you can do more. Maybe you can do something different.
If leveling in WoW is pointless (the point of the game is to reach max level and raid/pvp/scenario), then the point of leveling is to teach how to play, guide the player through story/narrative, and provide the player with time to get to know and in turn, fall in love with, their character. Leveling allows the player to consume more story, gain more things to do in combat (and out of combat, in the form of new resource nodes, new dungeons etc).
WoW is a multi-game. It is more than just an MMO. It is a community game, and exploration game, a challenge game. It does almost everything.
Mario 64, is an adventure game. It provides platforming as a challenge. Its what you do in it. The game rewards you with new an different ways to platform.
Anyways, back to WoW, the issue with the game is that leveling is not what you do at the end of the game. To change this, evolve questing to scenarios/dungeons and pvp. Each time the player completes a scenario, dungeon or pvp, allow them to gain new skills. To get something more, they have to finish different ones. The skill rewards could even be tied to the kind of play the accomplish.
Now instead of trying to get to level 90, you're trying to finish the scenarios and dungeons that allow you to have the skills needed to raid heal.
The key here is that newer MMOs need to look outside of pure exp/leveling mechanics to something that will fit what they want their game to do. If I was designing a WoW clone, I would seriously ask, what aspect of WoW are we trying to compete with. And if its raiding, then the question becomes, do we need levels? How else can we gate?