Making a game "playable"

I was watching my son play some online FPS games this week and I noticed something odd. Rather than the usual teenage rampage of shooting at, and blowing up, everything, they were playing a game of 'Simon Says'. A few days later I saw a group of players spinning in circles and randomly shooting.
Apparently the exposure to mindless online violence is fun for only so long, and what they were  doing was trying to find ways to keep it interesting. To me this indicates there are some shortcomings in the game design, but it also confirms that a lot of games have a limited shelf life.
One way around this problem, as my son and friends have already discovered, is to add extra rules or conditions. In chess this sometimes happens through keeping the game the same, but altering material (eg rook odds) or specifying some sort of condition (eg mate with a knight). Another way is to use different rules. The two most common examples of this are Transfer Chess (which is a radically different format) or Suicide chess.  Certainly at school clubs Suicide Chess seems very popular. I suspect one of the reasons is that it is far less complicated than normal chess, with a lot of the moves being forced. Also, capturing pieces is kind of fun for new players, so the more you do it, the more the game makes sense. The only down side I've seen is that a switch to Suicide chess is often a precursor to dropping out altogether. I guess if you don't find real chess fun, and you shift to a chess that very few people play seriously, it is only a matter of time before you move onto some other interest.