I haven´t played it yet, but the thing is that Civilization IV has an alarm clock: you can set it to beep after you´ve played it for a long period.
This is remarkable in more than a single way:
First, it´s a statement: 'this game is so addictive that we would be irresponsible to let you play uninterruptedly'.
It could actually be a request from the fans. How many game companies have to implement something like this? Most companies have problem keeping the player interested.
Second: The alarm clock is a reminder of the world outside. Most game designers like to stick to the (overrated and usually misinterpreted, in my opinion) 'suspension of disbelief' rule. Don´t let the user remember he is playing a game. Immerse him completely. Etc, etc.
Of course, you could argue that if the alarm clock wasn´t there, players would turn elsewhere to check the time. But the fact that there is no intention of completely hiding the existence of a 'real' time is remarkable.
The reason I say that is because, doing such, Civ IV acknowledges its audience as being adult and capable of, if not stop playing, return to it later, not having to keep them 'hypnotized'. It stresses how compelling it is and it allows players to fit the game better within their own life schemes.
I might be exaggerating, but I guess it shows maturity from the game and its players.
PS: This website is called 'nongames', and few games are more traditionally 'games' than Civ. However, the presence of the alarm clock is a feature that, I believe, would fit within the characteristics of nongames. The reason for that will have to be posted later...