If you' re curious as to what a real MMO is like you should try EVE. It' s not your watered down WOW or Guild Wars, ah no this is the real deal.
There is just too much to get into but as to why this is the best MMO and videogame with the most depth of all time. The complexity of this game is mind boggling. Only for the hardest of hardcore. For one thing, you only have one game server. Everyone is always playing on the same map. Which is HUGE, countless solar systems to get lost in. It' s actually the biggest game ever made, it makes Oblivion look really tiny. You can be anything between a miner to a scientist to a space pirate (raiding noobs) to CEO of a player driven corporation (you actually have stock in your corp.).
If you prefer to go at your own pace and just start out as a miner and collect raw materials to sell you can do that or you can take missions or work for one of the corparations/governments at war with eachother and get into these massive space battles with over a hundred ships.
There' s just so much to talk about in this game. I actually won' t bother much more. Just do yourself a favor and check out the site and try the free trial. In the very least you' ll get to witness the most hardcore game ever made.
right under the title on the first page you' ll see " try now"
http://www.eve-online.com/