So I will start with how I came to think of this; the experiences that led me to do so. I have been playing DotA, HoN, and LoL for around 1-3 years (depending on which). They are a game based on a formula people call Aeon of Strife.
[In it, one has allied, uncontrollable minions that spawn and attack enemy minions, trying to push down enemy defense towers, and eventually, destroy their base. During the game, one controls a powerful unit (hero) that gains power by being in the presence of enemy deaths, as well as being the one to hit the enemy minion or hero last, which gains money to buy items for more strength]
This formula has proved entertaining enough to me over the time I took playing and reading about these games. After some time, however, I got bored of the formula. Multiple times, actually. What kept me from stopping was a combination of obligation to do things with friends, and adding a few new features, like new items, new characters, switching to HoN, then to LoL, which has new of both, and new graphics.
[I played HoN about as much as DotA, but by the time I got to LoL, I pretty much stopped wanting to play seriously, and just wanted to win, or have good matches, which are incredibly rare because very very few people play at a consistent level. Which means 9/10-19/20 games are terribly unbalanced. The most fun games are balanced ones, but many games simply involve a weak player on one team resulting in the other team becoming overpowered and winning, as well as the breakdown of teamwork on the team with the weak player.]
The problems with the games only change somewhat when one gets into higher level games, which only happens after about a year of serious playing and improvement. Alternatively, one can find friends, which usually means that there is a team of friends winning many unbalanced games because they have more coordination in a team game.
So anyway, the last thing that kept me playing LoL was novelty, and a feature known as first win of the day. The first win of the day gives extra points with which to purchase champions.
This adds another element of obligation, beyond that of not wanting to disappoint
I find that many online games run on a level of obligation to keep one playing. MMOs encourage grouping, and then to disappoint one's "friends" feels bad. Or to want to get the next feature, whether it be an item, or a character, keeps one working at it. That is precisely what it becomes: work. Sadly, some of the most successful games (like WoW, Farmville) are successful because of this.
After the last vestiges of this have faded, games feel very pointless, hollow. One might feel that they have wasted much more time than they should have, because obligation rather than fun kept them playing.
I wondered; what is the point of playing games? Fun? What is the point of fun, of entertainment? In the end, it doesn't get anything done; it just passes the time. Is this acceptable? For some it may be, but others want something more. (I do) But what more is there? What can one do that is worthwhile in the end? What IS worthwhile? I don't know. I guess that's different for everyone.
Yet, fun seems to be worthwhile. It feels that way to almost all of us. I would at least argue that one should find the greatest degree and variety of this possible, though. In this way,
(so many times, I could close my eyes... :) I like when I think of some songs
one is exposed to a variety of ideas, invoking more creativity. What is useful in this world, if not creation?
[Well, anyway, that's my answer to the question. That the most entertaining things are worthwhile, and once one reaches a point where the things are just okay, one strives to fill the time with creating good or useful things, or interacting with people.]