gord0 That’s true. But the expansions come 2 or 3 years after the previous one so I guess that’s enough time for you to finish the main quests. Also, usually, the expansions have nothing to do with the previous story and it contains a whole new world with whole new storyline. Basically a new game. It’s up to you if you want to go through it.
The only problem I have with MMOs is usually that the games are insanely time consuming. In Aion, it takes you at least 2 months just to reach the level cap. So imagine playing a game for 2 months just to get to the interesting part of the storyline haha
Guild Wars 2 has done it right, though. Every 10 levels you have a story mission. In 80 hours, you reached the end game and you finished the storyline, plus, you have good enough gear to start a new story with the “new” expansion. And ANet is releasing the expansion every 1000 years or so. So it leaves you enough time to go through your Steam library twice before a new expansion is released haha