On launch day you're going to have tens (or if you're lucky, hundreds) of thousands of players all crammed into the starter zones. Three years later most of the active characters are clustered at the end-game, the low-level areas are sparsely populated by alts and the mid-level game is deserted. Game designers are still working on ways to keep this massive shift in population from ruining the game. (I'm interested to see how Blizzard's efforts to revitalize the low-level areas will fare.)
At launch the starter zones can be a frustrating crowd where people stand in line to fight over a single monster. Years later that same area will feel barren and lifeless, and new players will wonder why they seem to be all alone in an MMOG. At launch the large number of people gathering and crafting will reduce prices to commodity levels. Years later the economy will be distorted by the large number mega-wealthy characters that would rather just drop some gold at the auction house than acquire those items themselves. (As someone who tried World of Warcraft in 2008, I was able to break the economy just by selling my excess herbs at the auction house. Training, repair, upgrades and other supposed money-sinks were always trivial compared to my herb-gathering fortune. Then again, this might have been a distortion particular to the server I was on.)
If you haven't spotted it, I've been doing a humor series on Lord of the Rings Online. That game has aged well in a lot of ways. It's still beautiful, fun and a great way to tour an established fictional setting, which is always a treat for devoted fans. (I'm sure there are legions of fans would love to run around Hogwarts in a Harry Potter MMOG.) But one thing that has aged very poorly is the forced grouping quests.
In LOTRO, many quest chains end in a group quest. So, you'll do five missions solo and suddenly an NPC will tell you to sod off and find some friends. (And I do hope you read the quest text, or you might charge in alone without realizing you're committing a very humorous form of suicide.) You've just spent the last forty five minutes trying to find and save poor Gerebert, and now you realize all your work was for naught. You can't do it alone and good luck finding help.
(As an aside, I'm really against mixing solo play with forced grouping like this. You generally want to play in a group based on whether or not your friends are around, not where you are in a quest chain. Forced grouping takes the already complex business of teaming up and makes it that much more complicated. Where are you? Have you done these quests? Can I share this one with you? Are you the right level? Wait. Nevermind. We're both squishies and this quest needs someone who can tank.)