[Felix12g]

I should have been more clear, I'm not speaking of graphics when I'm talking about loads. AI behavior and other things that keep track of people in an area are what I'm talking about.

When you have people fighting on a battlefield, lets say in a pvp battleground, you don't have much strain besides the usual vectors for players and placement for rendering and targetting as well as the projectile and character information flying back and forth to the server. Vector calculations don't take much so any lag is most likely going to be client side and/or due to normal networking issues (Which is pretty much what Webzen is working on with their proprietary shit) All in all the servers can handle these things easily.

Now on the other hand you've got cities. Cities are filled with both NPCs and players. They've got a hell of alot more detail in the forms of advertisements, textures, buildings, decorations, stalls, vehicles and a whole shitload more. Beside the PCs and NPCs these are all client side worries.

Now, what you've got are NPC interactions and Players. The players don't mean a whole lot besides the load they put on NPCs and the actions they perform and just wasting space. A player may roam around the market district filled with npcs and other players. According to the interviews and other articles the npcs interact with you in different ways depending on your outfit, rank and other things. For instance an outfit's hangout next to some merchants will have those merchants acting different to the outfit, which includes more AI than figuring out vectors to paste a player in a combat situation. On top of this there's a whole bunch of players constantly roaming around, interacting with the npcs, other players and doing who knows what else. Add that to chat channels filled with people, people logging in, leaving etc and you've got a much heavier load.

The difference between a pvp area where there's players respawning and a town filled with persistant players crowding around and you'll have different loads. The servers will hopefully be up to handling the traffic.