(Wanted to start my own thread with a similar topic be decided to search the forum first, so a bit off topic maybe)
About a year ago i started a trial account for EVE. And i've had a few short bursts of playing it through the year. Bought a subscription a couple of times usually for several months played it a couple of days intesively and after that just got got enough of it. A couple of months later i would see one of these cool adds, or would read some stories about for example these intergalactic wars between alliances, and i would resubscribe only to stop playing again within a week or so.
At first the game would really hook me in. Got more money, better ships, better ships (probably comparable with new powerlevel and equipment in other games).
I think it's these rewards, like teletran3: said, which make you feel like your achieving something. This isn't a bad thing on itself; the same is ofcourse the case in other games, most games wil try to give you the feeling your doing something epic. The difference with MMORPG is the achievemants are a important motivator to playing the game allot. For example, a racing game is rewarding when you play it. You might unlock new cars and new tracks which is a nice bonus. But the game isn't less rewarding as a whole if you don't play it so often. In MMORPGs you are motivated to playing allot, because the more you play the more stuff/achievements you get. Even if your purpose is not getting rich and getting highest level character/ship, but to join an alliance (in EVE) or a guild, clan whatever or "the experience". You would still be motivated to up your caharacter because otherwise you can't join the good clans, or you can't really contribute to the group effort or keep up with your friends. This is maybe one of the reasons MMORPGs draw you in to spend more time playing it. It might be how the designers consciously set these games up.
But for me it doens't seem to work effective. At first it does and tends to become a bigger priority in my live, but at some point it's completely over. Sometimes it workes to well and i play till four o'clock in the morning leading to Ignignoct's Overdose and would be completely sick of it. Other times real live would prevent me from playing to an extend the investment i could put into it would be so minimal there wasn't any reason, for me at least, to keep playing the game. Usually at this point i would realize this was harming the rest of my live and wasn't really fulfilling.
(Disclaimer: this is my personal opinion, others might find a certain MMO a great facet of their lives and/or might be more able to enjoy with moderation.)
Currently I'm again a the third week of a three month subscription and haven't played the game for two weeks and probably won't do so for a couple of months.
