Allow me to begin by explaining the problem. Nearly every MMO has the same form of content it seems. You join, level for what may be a small to large quantity of time, then after reaching max level your turn your focus to gear. Gear which is effectively a permanent part of your character until you replace it with an even better piece of gear. So the main focus of the vast majority of MMOs is strengthening your character's stats.
Why is This a Problem (PvE)
I join a new game. My friend already has a high level character with good gear in that game. I want to play together with my friend. If my friend comes back to do low level content with me, everything will be far too easy for him and I will end up running behind while he steamrolls everything, not really contributing at all, while he is not challenged at all. If I go do high level content with him, he will be challenged but I will get one shotted by anything that notices me. If we do something in between I will still be useless and he will still not be challenged. There is no content appropriate for two characters of vastly different power levels to enjoy together.
Also, content now needs to be catered around specific levels. I have my level 20 areas and dungeons and my level 40 areas and dungeons. If everyone were on a similar power level then all content would be able to be enjoyed by everyone. You could even create specific areas and content to be extremely challenging and people couldn't just outlevel/outgear it.
Why is This a Problem (PvP)
I join a game wanting to PvP. Before I can PvP though, I need to slog through mass amounts of repetitive/boring PvE content to make my character strong enough. Or if I am lucky enough to be able to progress through PvP I need to get slaughtered over and over by high level / well geared character who have a massive stat advantage over me. I can't simply jump into the game and start having fun.
So What? If You Want Fairness Why Not Play an FPS/MOBA/Whatever...
MMOs are more than stat progression. MMOs are massive worlds that offer chances to explore the world, randomly encounter other players but peacefully, and in random encounter Open World PvP. MMOs can feature meaningful crafting, and a greater variety of content. MMOs give you a chance to make a mark on a world inhabited by real players. Some MMOs even have deep politics with player created factions. Some people love and desire to take part in many or all of these things without being subjected to massive power disparities based on level and gear.
But We Need Leveling, Leveling is Content
This argument died with the success of EVE Online. While EVE does feature character progression, character progression is simply selecting the skills you want and watching them level over time, whether you are online or offline. It's leveling provides no content whatsoever. What it does offer is a rich world with a player driven economy and rich storylines generated by interaction between players. It's not Alliance vs. Horde. It's Band of Brother vs. Goon Swarm and how those major factions interact with all the other player created and lead factions in the game. EVE proves the fact that if you allow players to generate their own content they will create a world more interesting than grinding for levels. And while you could argue there is gear progression in EVE that creates a lot of content, that gear progression can always be reset to nothing when you get blown up and your ship/equipment all get blown up with you. EVE could exist without skills, especially combat related skills, and still be a great game. EVE is one of the only games that is still able to successfully charge 15$ a month and it does this with one of the most boring combat and crafting systems found in any MMO.
That's a PVP Game. What About PVE?
Well, take a look at the original Guild Wars. A few days of hard play and you have your character at max level with max gear effectiveness. But there is so much game left after that point. You go around collecting new skills which are not inherently more powerful than the old ones, and cooler looking gear. Players still were motivated to go out and play past max level, and GW ended up being a wildly popular game despite the complete lack of an open world, any sandbox features whatsoever, or crafting. Features that could have made it even more compelling as a long term title.
We need more MMOs with less progression. Games where gear isn't permanent or level locked, and where we don't need to kill 1000 goblins to get the next level. Games designed to be fun from the moment you hop in and start playing, and that allow you to play with any of your friends and have fun, but that still have a rich MMO world to explore and take part in.