I worked for years on client and content side of a medium size MMO, closely with a colleague who was in charge for server side. It was successful in the sense of people paying a monthly subscription fee for playing.
Every time someone asks me for advice in this field, I just tell this: make a prototype of a very simple game (cards, board...) where you have a limited number of players. Take care of anyone wanting to cheat (i.e. sending crafted packets) pretending to have a service where your users value fairness and no cheaters.
Why? Because once you do that 99% well enough for a simple game, you realize the huge amount of work it takes for games of other sizes.