I'm going to start with a really, really tiny detail and try to use it to come to a sweeping statement about what makes a game fun. The detail we'll start with is weapon spawns in the Halo games: why timers are fair, but not fun, and why slightly random spawns may be unfair, but more fun.

History Repeating
Halo 1's weapons spawned in about the simplest manner possible: on a timer. Every X seconds, in a particular spot, a particular weapon would appear. Legend has it this was done because it was the simplest way possible. One can argue whether or not this is a good thing, but it is easy to understand, transparent, and about as fair as anything can be. Since everyone can know when and where the weapons will be, and the spawn is independent of whether or not anyone else is holding any particular weapon, then theoretically everyone gets, over time, a more or less equal chance to have any of those weapons.

This does introduce a sort of meta-game; a game outside the game. It's a game played with a stopwatch or timer, or with a specialized computer program made expressly for this purpose. It tells you, down to the second, exactly when each important weapon will spawn and where, allowing a player to be there on time, every time.

Of course, this isn't a "push X to win" scenario. Knowing where the weapon is going to be, and when, won't help if you get killed en route. It's not a substitue for the basic skills needed to play the game, namely the ability to navigate the map and the ability to aim a weapon and successfully engage in combat. However, all other things being equal, it does provide a distinct advantage over players who choose not to employ such a strategy, who will wander by power weapon spawns only to find nothing there, because they are too early or too late.

Remote Control
Fast forward to Halo 2's release in 2004, and its new, more complicated, and controversial weapons spawn system. For some weapons, namely the "power weapons" like the sniper rifle and the rocket launcher, the weapon simply will not respawn while someone else on the map is using it. If they're willing to compromise on the ability to carry and use two weapons, they can hold on to a weapon just to prevent its use by other players, and thus "control" that weapon, without the need for a timer. They can influence when the weapon spawns by when they choose to deplete/drop it, and that timing will be known only to them, since the game provides no notice to other players when a power weapon is dropped.

The spawn timing metagame wasn't removed, but only the player who had the weapon last could consistently play it.

Both games provide a means for one player to take advantage over another; the first, by superior knowledge of how the game works, either with or without external assistance, through accurate timing of the spawns. The second provides an advantage at the game from within the game.

In a way, the first method is a way of levelling the playing field. If two players spawn on a map and one is closer to a power weapon than another, when they meet, all other things being equal, the power weapon-wielding player will likely prevail. However, upon respawning, the second player has a reasonable idea of where and weapon that power weapon, or other power weapons, will be available, independently of the actions of the first player, and therefore has a chance to acquire one and thus even the odds in the next encounter. This means of levelling the playing field games from the metagame-- from outside the game itself. Regardless of how high or low your skill at playing the game itself is, you have a better chance of succeeding if you know reliably where and when you can acquire a wepaon superior to the one you spawn with, and that this information cannot be influenced by other players. They can, of course, also have such information and employ it, and attempt to show up and block your access, but the timing and location of the weapon itself cannot be controlled by a player.

First Mover Advantage
Halo 2's system, by comparison, focuses solely on the initial spawn position and proximity to a power weapon, and the relative skills of the players in deploying the weapons they have to determine who has the advantage. All other things being equal, the player who spawns closest to a power weapon will get that weapon first. If he or she is successful in using that weapon to its best advantage, they can prevent that weapon or weapons from being accessed by the other players. The initial advantage granted by the first spawn will be exaggerated over the length of the game.

It is perhaps ironic that while Halo 1 is often looked upon as the more skill-intensive game, that it provides a method to counter the wielders of power weapons with the simple ability to keep time; while Halo 2's focuses solely on the use of weapons. The counter to that, of course, is that Halo 2's power weapons (the rocket launcher in particular) requires less skill to wield but still provides an overpowering advantage. The counter to that is that there are many methods of countering the power weapons (weapon combinations, grenades, stealth) and so on.

However, I think there's an important reason why a simple return to a fixed, time-based spawn system would not be a solution to the ills of Halo 2's weapon spawn system; and I think the fact that it seems the Halo 3 Public Beta does not employ such a system is indicative of this.

Brute Force
As applied to cryptography or game-playing, the concept of "brute force" refers to the application of the quick evaluation of all the possible permutations in a system to reliably arrive at the optimum. In simple systems, this can be done quickly, as in games of tic-tac-toe. The game's depth, it's replayablity, is inversely proportional to the degree to which it is susceptible to being solved by brute force. As it is trivial to arrive at a simple set of rules that describe the optimum move for any player in any game of tic-tac-toe at any time, and that failure of any player to deviate from that game plan results in an endless series of stalemates, the game is essentially broken.

This can also happen in far more complex systems. For a long time it has been speculated that eventually, computer systems will reach the point where even deep games like Chess or Go might be solved by brute force; where the ability of a computer to evaluate all the possible combinations of moves from any initial starting point may result in the "perfect" game; a proscription for a series of moves that will always result in a victory for the side moving first if followed correctly, or perhaps where the best result that can be hoped for by an opponent moving second is a stalemate.

A computer game like Halo, while vastly different from a turn-based game like Chess, can also be susceptible to being "brute forced" as players develop a series of actions that are extremely difficult for opponents to counter and consistently give good results, almost making such actions within a game mandatory.

A weapons spawn system that functions solely on a timer is not only susceptible to this, it practically demands it. A Halo player who is unaware of the spawn timers is handicapping him or herself. So the question for the developer becomes whether or not they want this willingness to use a timer to be one of the skills that playing the game is supposed to take into account; whether the ability to use a timing system, and the willingness to follow its edicts in order to maintain weapons superiority, is a trait that should be rewarded by the game.

Out Of Chaos Comes Order
The best solution is most likely dirty, controversial, decidedly non-transparent, but likely immune to brute force: randomization. Spawn power weapons at random intervals. Spawn power weapons at intervals that are sometimes randomized with reference to when they were last spawned, or to when they were last dropped. This actually gives players more choice. They aren't forced, if they are within range, to check a spawn point for a power weapon just because they know (and every other player knows) that it will become available at that point. Neither are they prohibited from having access to a weapon merely because another player is still using it. If they are near a spawn, they have to measure the potential benefit of having the power weapon against the risk presented by traveling to that spot and the risk of the wepaon not being there. Because this is not a predictable system that can be brute forced, there is not a single sequence of decisions that will always lead to victory. You can make the same choices in a different game, from the same initial position, and get a different result because of the random element.

Nobody said it would be fair; but perhaps the point is that fair and fun don't always intersect.