6. Bad Captains
To be fair, some people probably don't know how important the captain position is. However, a team with a good captain will already have a distinct advantage over a team with a bad captain before the match has even begun. Again, this will most likely effect only those who play Virtual Pro and Team Play, but it certainly holds true to any of the game modes. Be a bad captain and you're likely to lose. Captains get to control the team orders, formation and play style of the team. By default, the 'neutral' starting position of teams means the A.I. will be crap at everything. Defending and attacking. By simply choosing 'attack' or 'defend', let alone 'all out attack' or 'all out defend', you'll already gain a foothold over a captain who does nothing as the A.I. actually becomes somewhat better at the game. Take into account all the other things a captain can do (like kick the goalkeeper out if they're running around like a headless chicken), you can see why this position is vital to strategy. Unfortunately, many players just leave everything as is and make no adjustments whatsoever.