There seems to be a critical mass for fun, particularly where playstructures are concerned. Take, for example, your neighborhood playground. If you swing by early on a weekday morning there's maybe one or two kids there, if you're lucky. Even if they are older, everyone gets to go down the slide as often as they want, stay on the swings for as long as they want. And they burn through the excitement of going to the park in the space of half an hour or so.
Now take the same park and stuff it full of kids. Maybe a hundred kids and throw in a few watching parents to facilitate problem solving. Make them wait in line for the swings, the slide. Make them negotiate who gets the blue shovel in the sandbox, make them decide which of the drinking fountains is "pee" and therefore verboten.
Add all this low level conflict and interaction to the mix and suddenly you have an experience that is a thousand times more engaging, a thousand times more *fun* even with the exact same equipment.
This same effect carries over into adulthood. You never want to be the first one at the party, not because some 60's social maven suggested it was poor form, but because it's just no fun until the room reaches a critical mass.
So what you seem to be seeing (I think) is a built-in effect. People, by and large, being somewhat social creatures by nature or design are looking for a packed house. When you log onto your MMO of choice, take a look.
Why are there always some servers packed to the gills even though there are half a dozen others virtually (pun intended) empty? These people all must have seen the same thing you just did, they didn't all log in instantly. Even if you/they have different characters on different servers, many people chose to create those characters on the packed-in servers.
Despote the grousing about epic drops being rare or servers being too crowded, I can't help but think that you are shooting yourself in the foot by expanding those elements too soon too fast.
There is an equal critical mass on the other end, where we all devolve into snapping snarling primates fighing over an Epic Sword of +21 badgerslaying, but judging that tipping point is going to be key, you want to move soon enough to keep people from leaving the game because they have gotten stalled in their progress, but not so soon as to empty out the space and take your product back down to that empty playground.