Cheater clients are more of an issue in MMO than multiplayer, usually multiplayer is with friends unlikely to cheat, but even with stranger lobby servers the impact of cheater clients is localized to those few sharing the server. In an MMO cheater clients have a global impact in competing play (PVE or PVP). So it is not just an issue of scaling up your server to handle N connections, it needs to be NxM so that everybody can see everybody elses customized avatars moving around properly, and you have to remove client cheats by doing most everything on the server so that the client is nothing but a display. This is why most MMOs are still only instanced zones, even though it might be a megaserver you still see only a few around you, and that is to keep the order of the server tasks manageable and parallelized (besides they fact that nobody wants to see a million people in a medevial town anyways).
Though i have to say this Morrowind MMO nightmare was very prophetic of what ESO ended up looking like.http://static.giantbomb.com/uploads/ori ... ls_mmo.jpg
This is a critical MMO security design that ESO got wrong BTW, they killed the bots by nerfing alt recreation that commercial gold farmers needed (forcing them to steal accounts now so they moved on to more lucrative targets), but cheaters can still take advantage of it and stay under radar to avoid getting their alts reported/deleted. Since ESOs focus is on AVA they mistakenly said we want 100v100 battle (which is a server order of 10000 not 100), so they took server shortcuts to get that order down without compromising the battle by leaving much stuff on client side. That of course can be intercepted and changed, their PVP forum is full of complaints about cheaters since you can easily find the cheater clients with a google search. But how does support really know if you got victimized by the lag monster or a client cheater, it is actually hard to enforce short of admins putting you on 24/7 follow watch, which needs a lot of $15 accounts to pay for finding that one cheater which is hard to do when people are quitting because of cheaters!
The other issue is game balance, how do you balance against the unemployed that make the MMO their day job grinding thru content that should take six months to complete in less than a month, without making the game so hard that the other 99% rage quit. This means dynamically add content in a transparent manner compared to the often nightmare modding mess that single player games have (you can read lars blogs how hard that has been to not make it a mess!)