It seems to me there are some common end game conditions, which can be categorized by a few factors:
1.)
Controlled/Uncontrolled - how much control do the players have in creating the situation?
2.)
Winning/Losing/Ending - does fulfilling the condition simply end the game, or does it also make you a winner (or loser)?
3.)
Known/Unknown - Do you know at the beginning of the game when the game will end?
So, lets look at how each of these factors changes gameplay:
An Uncontrolled, Ending, Known end of game mechanic is similar to most games that end in a set number of turns. Basically this turns the game into a production race with a fixed end point, where you must do the most X in that time. Optimization is very important, since your opponents have the same amount of time as you, and you all know how long that'll be.
An Uncontrolled, Ending, Unknown end of game mechanic is similar to when a random "end of game" card is put in a deck, and when it comes up, the game is over. This is similar to the above, in that it's still a race, but now you have to take into account that you don't know how long the game will be, so longer term strategies might not have time to pan out.
An Uncontrolled Winning or Losing game is kind of the degenerate case. You have no control over the end of the game, and the end of the game decides who wins or loses. The game basically is a slot machine. Pull the handle - did I win? No? Damn.
A Controlled, Losing, Known end of game mechanic is similar to most "eliminate your opponents" games. Note, I call it losing, because it's not really that you win the game, it's that everyone else loses. In this kind of a game, there are several strategies - go defensive and try to let the other people kill each other - go offensive, and make yourself so dangerous, no one else wants to attack you. Alliances against a strong opponent in this kind of game are common.
A Controlled, Winning, Known end of game mechanic is similar to Illuminati. do X, and you win. Similar to the first example, this makes the game into a race, except that now the endpoint isn't fixed - players determine when the game ends. This kind of game can end quite abruptly, but usually you can see it coming and can even work to prevent it from happening.
A Controlled, Ending, Known end of game mechanic is similar to any game where doing X things will end the game. This is a very interesting mechanic, because it means that even if you can end the game, you don't have to if you're not in a position to also win.
Controlled, Winning, Unknown ending mechanics games are common in games with "you win" cards. These games tend to end very abruptly, with very little chance for opponents to react. Fluxx is a good example, as are alternate win condition cards in Magic the Gathering. In these games, the end of the game is almost always abrupt, and can feel Uncontrolled by anyone other than the person who wins.
Controlled, Losing, Unknown ending mechanics games are fairly uncommon. There may be some examples in Magic the Gathering, but few others. This is a particularly poor choice, since it makes the end of the game very abrupt for one person, and if the game is multiplayer, that person is likely to feel picked upon and left out.
Controlled, Ending, Unknown - similar to the uncontrolled version, generally some token you draw gives you the ability to end the game. The fact that it's controlled is almost worse, because then it's one person's choice, but no one else knows they have that choice to make. Not very fun.
So there's my matrix and my take on the options. You can extrapolate this to fit just about any game.. and of course, many games will use more than one of these, and many fall between strict categories, especially about how much control players have.
