[rantish dissertation]
At all times in a grind-ladder game, there are uber-players who pwn you, and n00bs whom you pwn.
You feel hate and rancor toward the uber-players, and secretly think that they are all lifeless 11-year-olds who play the game 16 hours every day, and thus are beneath you. You feel nothing but contempt for the n00bs, and miss no opportunity to remind them that they are dirt and should GTFO.
At level 5, level 10 players are 11-year-old losers who will never have a date and level 2 players are helpless idiots who couldn't quest their way out of a paper bag. At level 50, the level 60 guys are the ones who blow all the money they make bagging groceries on hours and hours of internet cafe time, and the level 30 players are total smacktards who couldn't get a clue if you carved it into their ganked corpse.
And so it goes, an endless treadmill with a carrot dangling in front of it and the player, like Tantalus, strives eternally for a goal that is forever just out of reach. The goal, of course, is to have EVERYBODY else in the game fall into the "clueless n00b" category, and have you, the character, be to them as the hero of a single-player RPG is to NPC enemies, wading through throngs of them, laying about with your sword and spreading death in all directions.
But you never get there, and eventually you have to settle for
other forms of amusement, until finally you just quit playing the stupid game.
[/rantish dissertation]
So my advice is to have a clear point in the game where you cease to climb that ladder, stop walking on that treadmill, and crystallize your place in the game's hierarchy. Here's how you do it:
Have the first five hours of the game be grind. Not just any grind, but a min/max tutorial grind. What you do during this time will determine your character's strengths and weaknesses. Where you go will decide what vulnerabilities and resistances you have. A handy tip feature will teach you about game dynamics. Then, after you finish all this up, you "graduate" into the real game, which is a vast plateau instead of a summitless mountain.
Your character is defined by a five-hour grind, and now he's just about done growing. The rest of the game is like other games at level 60: You get together with other players and decide what will be fun. Then you do it. If you decide that you need a medic to get through a particular set of challenges or attain a particular goal, then you start an alternate character, and in five hours you have a totally serviceable medic that can actually help your buddy's three-month old commando get through the robot-infested ruins of Technopolis. Or you just find someone who's already a medic and invite them to come with you.
The problem here is that you run out of content. You don't have the hierarchy of monsters to constantly challenge the players, but that dynamic has always sucked(Don't go over that bridge! The beetles there are yellow, and tougher than any bear you've ever seen!). It was stupid in Dragon Warrior, and it's stupid in World of Warcraft.
Instead, have a reasonable and intelligible system. The really badass monsters are rare, because they need a lot of territory to hunt and feed themselves. There's no reason for 5000 Lions to be able to coexist in a little forest, and there's even less reason for one guy with a ninja sword to be able to kill six of them at a time. You want to hunt lions, you get a group of guys with spears and go hunt some lions. No amount of toad-stabbing will make you able to take a lion in a wrestling match. That's also a stupid relic from a time when incrementing numbers was the only way to show growth in a video game.
PvP will make some areas unapproachable. EVE's 0.0 security space is a free-for-all, and you stay out of Faspera because there are some badass pirates living there. THey don't guard a secret trove of treasure, and they don't have the key to a magical chest. They play EVE in the evenings and enjoy killing you. That's why you don't go there unless you are friends with "Swisher". You are not.
Make an MMO world that can be inhabited, and in which an organized society is worth more than a fifty-two-hour sleepless bender.
These people are paying by the month, and having them on 24-hours a day doesn't make you more money, it just causes lag. Have "powerlevelling" be passive. If you want to boost your shotgun-shooting ability, you get a shotgun and some ammo and practice. Tell your guy to train for a few hours, then turn off the game and go to bed. In the morning, you will be out of ammo and have spent $500 on range fees, but will get a +3 bonus to your shotgun skill. This will let you aim faster and hit targets more reliably. It will not make your shotgun destroy tanks.
Whew. I'm tired now.