Balance

 

Arguably the most important factor in game design. It appears in many forms and is often demanded from the developers by fans of games. It is important(especially in multiplayer games) to make sure there is an environment to begin a game in that offers no player any distinct advantage over another. That isn't to say that there is nothing in the game that will help a player gain advantage, the point is that the advantage is viable for any player to obtain, even if they have to compete for it. 

There aren't actually too many games that are perfectly balanced(if any at all), but there are games that strive for it. 

Starcraft is a Real Time Strategy game with three distinct armys to choose from. A human based army, very similar to the vision of space faring humans from the Alien film series; drop ships, marines and attitude. An alien race also similar to the aliens in the Alien series. A hive mind race that hatch from eggs and morph into creatures and structures. Another alien race that has a technology far in advance of the others. What they have in technology and combat strength, they lack in numbers. The designers are constantly updating the game in order to find the most balanced playing field while keeping the different armys... different but balanced. This is often found lacking in titles and as a result, they do not retain fans for any length of time. 

Popular games that had/have a well balanced(and updated) design.

Starcraft 1&2, Quakeworld & Quake 3/Live, Counter-Strike & CSS.

There are possibly a few more, but chances are it could be argued that they don't strive for balance.

Balance is connected in many ways to the over all development of a game. It will return in topic again.