WOW is in many ways a patched together hybrid of 3 different games: Levelling, PVE and PVP. There are people who enjoy all 3 parts of course but generally people end up spending the vast majority of their time in one of the 3 areas. The most vocal players on the forums and on websites tend to be those that focus on PVE and PVP of course because they are generally the more hardcore groups focused on perfection but there is a huge portion of the playerbase that totally ignores the endgame and just levels character after character up to the level cap. The trouble with the current setup is that people who have no interest in levelling up characters and seeing the various zones and stories are forced to do so at least once and then again any time they want to use a new character class and those that just want to level up alts get to watch the endgame vacuum up nearly all development time. There are huge numbers of other games that have competitive PVE or PVP content that do not have this requirement (like first person shooters, say) and making levelling mandatory prior to engaging in what is the main part of the game for many people seems like something that isn't necessary. Could we not create a game in which people can go out and do quests and see things without forcing everyone to do that for a long time before engaging in the rest of the game?
I think the first step in that direction might be to remove levels completely. Characters can gain skills and powers without level coming into the equation at all and this would mean that characters would not be utterly unable to fight monsters in much of the world when low level and also would mean that characters could not obliterate the weaker monsters so easily once they are at the level cap. This also means that when a new set of content is released players do not have the expectation that they can gain more levels and with those levels gain massive increases in power. You have a choice to make at this point though: Do you allow characters to gain in power internally or are power gains going to be entirely gear dependent? If characters get more powerful internally up to a cap then everyone still has to go out and do whatever it is you do to get more powerful until they are capped before they can really get into big group endgame content so we aren't any further ahead. If characters can't get more powerful themselves but merely can put on better gear then getting into the endgame is very fast and a good player would be able to be very competitive in no time. Unfortunately that has the side effect that people don't *feel* like their character is improving but rather just that their gear is getting better - they are just a vehicle for their equipment.
One idea I have been tossing around in my head is a rolling, unachieveable cap. Some MMOs do this now, but none of the ones that are big in North America do, it is a feature found in some Asian MMOs. The idea is that the level cap is 90 but that nobody in the world can reach it because the last 10 levels each take months to get. There will be some people who get to 85 or so but the majority of the world is going to stop levelling at 80. When you want to release new content you just bump up the level cap to 95 and push back the point at which levels become brutal to gain by 5 levels also. We would also need to remove level from the combat equations. The idea is that there is always an incentive to go out and do more quests or dungeons because your level can always increase but that there is a fairly small difference between the absolute top player and someone who plays much less. It would also mean that a new player is weaker than veterans but they can jump right into their chosen mode of play very quickly because their level is not nearly so impactful.
The idea is to dramatically lessen the impact of each new piece of gear and each new level so that a new character has to do a fairly small amount of questing and learning before they can start playing with their friends and trying out all the parts the game has to offer. If a level 85 character has double the health of a level 30 then the level 85 is much more powerful but at least they could do some things together... when the level 85 has 1,000 times as much health and can defeat anything with one attack then they cannot usefully cooperate. This would end the tiered raiding style of WOW because gear would not be obsoleted nearly so quickly because it wouldn't scale very much - a group of people might well be able to raid half a dozen instances that could be considered reasonable challenges with useful rewards. It would also make swapping characters in PVP much easier as getting to the baseline for being really competitive would be much quicker, though becoming the absolute top would remain utterly brutal.
Whether or not this style of game would actually be more popular than WOW is not a question I can answer. Lots of people talk about what they want in a game but they rarely understand their own desires well and they often refuse to play games that have exactly the things they claim to want. Maybe the mandatory level up grind is actually really helpful in retention... I don't know. I would like to try this sort of game though as it has a feel much more like Diablo 2 than WOW and that might work out really well.