A huge part of current MMOs is only there to guide the players to the maximum level where the real game takes place. I know I myself usually rush to the new maximum in WoW whenever an expansion hits. I don't care about the content in between; I don't care about crafting (other than getting that to max level too,) etc. Keen calls this “concentrated action” in a follow-up post. Instead of having a lot of things to do all over the place, we are funnelled into a linear progression path to the endgame.
Many players see this as natural, but it wasn't always this way. In Ragnarok Online, an obscure Asian grindfest I used to play, only very few people were actually at the maximum level. You could already participate in all content below that level and getting to the maximum was really hard. (Or, well, really grindy.) Also, things you achieved before getting to the maximum still had an impact on your character - unlike gear in World of Warcraft that just gets replaced anyway. Why is it that these days we follow one progression path (levels and experience) until it is capped and then enter a different path (gear) once the real game starts?
Keen's solution is to make gaming more horizontal, giving the player more things to do while levelling and making the whole levelling process slower. I think that that, while not incorrect, fails to address the real point. There is a huge part of the game that is of little interest to many players that they have to play through to get to the interesting parts of the game. Granted, not everyone is an endgame junkie and many players actually enjoy levelling (so much that they do it over and over again) but many are. The two games have hardly anything to do with each other - levelling has nothing in common with raiding and vice versa. This, to me, is really odd.
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Two major differences can be identified between the system in WoW (and its clones) and the one in Ragnarok Online. For one, low level monsters could actually drop good loot in Ragnarok. There was no concept of binding in the game and in-game currency was used to facilitate trading of items among players. That means that finding an interesting piece of gear (usually a card1) wasn't just useful to you while levelling but you could also sell it and acquire other items with the money you earned. Fast levelling had to be weighed against slower levelling with more interesting drops. In WoW, you would always choose the faster levelling since whatever drops you get during levelling will be next to useless when you keep on doing it.
The other difference is that WoW's endgame content is artificially limited to maximum level players. There are pretty harsh level requirements on the dungeons and items themselves as well as mechanics such as crushing blows that make fighting a higher level mob unwise. Ragnarok Online had no such limitations - if you were powerful enough you could go fight wherever you wanted. That means that a group of lower level players could still go out and try to kill a boss - obviously with lower chances of success than a group of high levels. In WoW, such a design would be nigh-on impossible. Either the lower level group would be unable to kill the boss or it would be trivial for the higher levels. This is caused by the huge difference levels make in WoW. The difference in strength between a level 78 and a level 80 character in WoW is huge, while in Ragnarok it is barely notable.
One part of WoW's success is the instant gratification that the players get. Each new level makes you so much more powerful which is then counteracted by throwing stronger enemies at you. Lather, rinse, repeat. Taking that away in favour of a more gradual progression path might turn players away. This is a tough game design decision to make, but I think that the existence of an easily attainable maximum level and the focus of challenging content on players at that maximum level make level progression irrelevant.
In an ideal world you would still have very challenging bosses in the game that require you to be of a high level (due to their difficulty, not artificial limits) but you would also have less challenging encounters that still drop relevant loot. Higher level players could still have a challenge there by going with fewer players (getting more loot per person in return.) This goes somewhat hand in hand with my call for more interesting gear, as WoW's current model hardly allows for a level fifty drop to still be useable at level eighty.
This goes beyond gear and experience by the way. Games that have a variety of skills (feats) that are trained through their use allow for relevant levelling content. If you ground your way up to maximum level on ogres, you would likely be well versed with your daggers and know ogres in and out, but other skills would be lacking. There are slight hints of this in WoW, but they are not really fleshed out. Reputations used to make a difference depending on how you levelled, for example. If you chose to level to sixty in Winterspring you would be behind in the race for Argent Dawn reputation and if you skipped Zangarmarsh you would need to grind more Cenarion Expedition reputation before being able to repair at the entrance to Serpentshrine Cavern. Weapon skills also work this way, if you used maces to level all the time you would be unable to use swords effectively and vice versa. All these examples just result in additional grind, however, and the effects are minimal.
It is a lot easier to design content for WoW's model since you don't have to care about the impact of levelling content and you know exactly how strong the players that challenge your bosses will be. In the end, however, the existence of the endgame kills levelling. Now, I'm an endgame raider and I don't want that to be gone - but why does it have to be endgame? If the levels were closer together in power I'd be fine with taking a mixed groups of level 75 to 80 players into Icecrown. Hell, why not allow a group of fifty level fifty gnomes to try it? If they manage to kill a boss, they get some loot but will have to split it among 50 and not 25 players.
1 Monsters in Ragnarok could drop cards that could be socketed into items. Each card would have a unique effect depending on the monster it dropped from.