Ok, I think I was wrong puting the fault to the level 1 start. The real problem is that the leveling process cut the multiplayers interractions greatly, because it got stucked at a solo conception of leveling. Nearly every MMORPG made this fault.
Adding more quests could make the leveling "less unfun", but the cut in interactions would be the same. Group quests are a false solution, because it gets the interactions at the level of a multiplayers RPG, not a massively multiplayer one.
Some MMORPG partially avoid the problem by changing the way of leveling. In EVE Online, the leveling process is made by skills selectioning that are upgraded through IG and IRL time. Like an upgrading plan.
In Ultima Online, the upgrading was made by using skills. Unlike Dungeons and Dragons original system, you don't win experience points by achieving monsters and quests, every movement using a skill contributes to upgrade it.
This way, you are not condemned to isolate yourself to level, your battle interactions with other players upgrade your skills too.
In a NeverWinter Nights 2 server I was on, talking (Role Playing communication) was making the player gaining experience points.
These are some ways to keep a leveling system without cuting the interactive experience.
I think the real way to keep interactions between players is to promote Role Playing actions. By Role Playing actions, I mean all actions that have a long term influence to the world.
We call MMORPG worlds "persistent" and this caracteristic is the main interesting thing about them.
I think instead of starting to level 21, the leveling system should evolve to something more inovative and MM oriented.