Team Play
The system should accentuate the strengths of the group dynamic as a whole and should minimize the weaknesses of the group collectively. Of course, training time can be devoted to working on the weaknesses in order to improve upon them, and, time can be given in the effort to stabilize a team's strengths. In the mean time, a system can cover up the rough spots, and claim advantage to your strengths.
Developmental Levels
The decision should take into account the developmental level of the players. In other words the sophistication of the system should take into account the abilities and level of comprehension that the players posses.
It makes no sense, with the younger players, to devise complex plans of attack based on overlapping attacking players and a clever interchange of positions when your team does not claim the technical expertise to carry out the plan. Soccer is a simple game and the players should be able to recognize it as such. Keep to the basic principles and do them well.
Execution
It should be remembered that systems do not win games, it is the player's ability to carry out the plan that wins the game.
Outside Factors
Choice of the system can be based on the environmental conditions that the game is played. If it is raining hard come game day, and, you find yourself on a field that is less than average, playing with a sweeper behind the defense may be a good way to approach the game, and, putting a fast player on their back to force mistakes in the difficult conditions may be a good adjustment.
If a team is used to playing with five midfielders and finds themselves playing on a very small field where they are just getting in each other's way, perhaps playing with 3 or 4 midfielders instead may be a good choice.
Experience
With the younger players--U-12 and below--the major consideration for the coach should be to teach the "principles of play" instead of a formation for results. Make sure that players understand the principles of support, cover, creating space, pressure, depth, etc. Teach these first, using small sided games and activities that challenge players to solve a problem on the field.
With the U-6's and U-8's, a formation is really not a huge part of their play. Having fun and coming back next year are the major goals. A formation for these youngest players is a place to start the game. But, once the whistle blows, forget about maintaining a balanced formation.
With players that do have a bit of an idea, a realistic expectation for some U-10's, our job is to keep things real simple. It may be a good idea to keep the field balanced. For example, if you are playing 8 v. 8, play a 2-2-2 and take the one additional field player and let them be a rover, going where the game tells him or her to go. If the game is balanced, the person plays in the midfield. If the team is struggling, the rover may end up on defense. For 11 v. 11, play a 3-3-3 then have the rover go where the game dictates.
This is not an exhaustive list by any means. However, it does give us some things to think about, and some principles to adhere to when choosing a system of play for our teams. It is important to remember that overall, the coach needs to make decisions on what is appropriate for the team. We need to adapt to the players, instead of having them adapt to us. Choose a system that fits them. Then, we can refine and interpret the smaller points to make us effective as a team.