The biggest issue is actually likely going to be the fact that a MMO is designed to be played by a lot of people at once.

Take a game like WoW, if you could make it a single player game you could still do questing and stuff but dungeons and raids would be out of the question until you vastly over level them because they're designed to be run by groups of 5 to 20 people. They're also designed so that the groups need to have tanks, healers, and damage dealers. I now also no longer have access to the auction house to buy stuff from other players, especially crafting materials gathered from professions I don't have because I can only have 2 at once.

So to make WoW a single player game, you'd have to change the core fundamental design of the game.

From a technical standpoint the server in a MMO tends to handle a lot of the AI scripting, loot drops, storing of all character data, and general multi-player stuff.