I hate leaving a good MMO and once I find one I tend to stick around for a couple of years. However, all games eventually grow old or the developers start to get lazy and it becomes time to try out new things. This year with Age of Conan and Warhammer both coming out I have a feeling one of them will replace my current favorite. In fact I think quite a few people might migrate from an older MMO this year to one of the newer offerings. And I bet most of us will have a similar problem, "How do we get our friends to follow us?"
Even if you convince your friends to migrate to a new MMO with you there are still a ton of rookie mistakes that can be made. Almost no one wants to play a class which was similar to what they played in the last game. This of course means that your previous group make-up has to be redefined. Leave it up to chance and then laugh your butt off when all your friends login as rogues. You might also want to establish some ground rules for leveling, questing and joining guilds. No matter what happens your group will eventually split apart but with some good planning you might be able to make it so that it doesn't happen until the end game.
Don't force friends to play classes they don't like
Make sure the new class types for your friends match their personality
Only try to move friends to a "new" MMO
Migrating only works when friends are tired of the current game
Keep an eye out for those power levelers
Guilds will break up your friends group
Guard your Healer