1) The main characters need a reason for becoming the heroes. If the person playing the game ever questions why a certain character is sticking through numerous bad situations, then the believability of the game becomes compromised.
2) The main characters need to be appropriate for the kind of story that is being told. You can't just come up with any kind of character and make him or her the central hero of any kind of game.
The reason I decided to write about this topic today is that I was recently reminded of Final Fantasy XII, a game that had great potential that was held back in part by a poor choice of main characters. At this point, I should put the disclaimer that I did not finish FFXII. I estimate that I made it about half way through the game. However, even though I made it that far, it still had not become apparent to me why Vaan, the supposed main character, had even joined the team.
At its core, the main plot of FFXII deals with war and the political situation concerning Dalmasca and the Archadian Empire. Even though the main character is technically Vaan, the princess of Dalmasca, Ashe, is the character who actually drives the action of the game with her desire to see her country freed from Imperial control. Balthier and Basch also help drive the action because they have personal connections to some of the most important players in the game's action and are sympathetic to Ashe's goals. Fran and Panelo join mostly out of their connections to Balthier and Vaan respectively.
The character whose motivations are questionable is Vaan (and by extension Panelo, who he drags into the events). The reality is that Vaan never really has a motivation to save Dalmasca. Unlike Ashe or Basch, he doesn't really seem to be very patriotic. His only tangible connection to the political events of the game come through his brother, who plays only a minor role in the game that is overshadowed by Basch. His main connection to Ashe is that he can apparently share the strange visions she has, but that remained completely unexplained half way through the game.
Early in the game, when I was first getting to know Vaan as a character, his strongest motivation is to become a sky pirate, the same as Balthier. However, Sky Piracy has very little to do with the plot of the game. The game doesn't even dwell too much on what Sky Pirates do other than fly around in airships. Vaan's obsession with sky pirates has more to do with the allure of freedom from responsibility and his current worries in life. In that sense, Vaan is the archetypal protagonist of a coming-of-age story. Unfortunately for him, FFXII is not a coming-of-age story.
The is really no room for Vaan's story in Final Fantasy XII's plot. A lot of the time, Vaan's lighthearted chats with Panelo and such are pushed off-screen in favor of more serious dialogue involving Ashe and Balthier discussing the morality of using ancient super-powerful artifacts as weapons against the Empire, and how much they actually trust each other. Not even the other characters take Vaan seriously. Balthier quips that he brought Vaan along for entertainment. The one time the other characters ask Vaan why he is coming along, he ends up grasping for something to say, and they end up just walking out before he can come up with a response. It seems that even the game developers did not take Vaan seriously as a character.
It might have been better if the developers had picked a different character to be the lead character. One good pick would have been Larsa, the prince of Archadia who drops by the party often enough that he might as well be a party member. Not only does he have direct connections to the politically heavy plot, he is the main villain's little brother. He also demonstrates a very pro-active interest in the unfolding events of the game. He is also young enough that he could easily take on a lot of the coming-of-age plot aspects surrounding Vaan. Not to mention that he probably has more speaking lines than Vaan anyway.
Vaan's weakness as a main character could become a more long-term problem for Square-Enix. Many of the recent Ivalice Alliance series of Final Fantasy spin-offs, most notably Final Fantasy XII: Revenant Wings, have been using Vaan's appearances in those games in their marketing to try to bank on the popularity of their big budget game. However, since Vaan put in a very weak showing in FFXII, his cameos and second appearances might fail as an effective draw for players. Weak protagonists lack long-term marketability.