Deep in our hearts we all know that the MMORPGs we play are just games. The dragons we slay aren't real, nor are the epic treasures we gain. MMORPGs are a fun way to spend your time, but few people mention their virtual achievements on their CV. Talk to somebody who doesn't play World of Warcraft and tell him how you did slay Illidan, and instead of any admiration you'll just get a puzzled look. That is not to say that to those who play, the virtual adventures and achievements aren't important. There is a concept of suspension of disbelief in which the audience knows that what they see isn't real, but is willing to forget that knowledge temporarily in exchange for being entertained.
The problem is that this suspension of disbelief isn't equally strong among all players. That was pretty much evident when we discussed PvP servers in WoW, where some players were talking about how PvP added more danger to their virtual lives, and others resented being ganked by pimply kids. Same situation, totally different perception, totally different levels of suspension of disbelief. And of course that can lead to problems with the success of a game: what if your game is about impact PvP, and you aren't offering non-PvP servers? The players who suspend their disbelief strongly will delight in the "impact" their PvP has on the virtual world. The players more strongly rooted in the real world will experience the same situation as being more disruption to their game style, and won't like that at all.
Suspension of disbelief is a good thing to a certain extent, as it allows you to overlook the shortcomings that every form of media has. You sit in a chair and don't really move all that much, but still manage to believe that you are living an adventureous life killing dangerous monsters and gathering treasures. If you see the rewards as just a bunch of pixels, you will be a lot less entertained and enthusiastic. But this suspension ends at some point for most people. A lot of people for example don't raid for the simple reason that in their perception the real world effort you need to do to raid (playing until late, staying in front of the computer for X hours straight, etc.) is too high compared to the value of the rewards, which is only virtual. Thus their comments about raiders having "no life" [insert various South Park jokes here]. Objectively spoken it is hard to say where exactly the sane border is. Some people need less sleep, and their personal situation allows them to raid every night until midnight with no negative effect on their work or family life. Others crossed the border into what is commonly called "addiction" (although I don't really like the term), where the suspension of disbelief is so strong that the virtual rewards become so important that the real life suffers. Where that border is isn't necessarily the same for everyone, you can't measure it in hours per week played or anything. "Hardcore" is jokingly defined as "somebody playing more than me", but the joke is only funny because so many players think exactly like that.
Developers are well advised to think about how their various game features appear to players of varying levels of suspension of disbelief. What is often called "casual friendly" is just that: making a game for people who haven't suspended their disbelief all that much, and can't overlook things like anti-social behavior from other players, or harsh time requirements of a game just as easily as some others.