Most people vote based on pre-established party identity or single issues. The main problem is that a two party system ends up with the parties being opposed to

Most people vote based on pre-established party identity or single issues. The main problem is that a two party system ends up with the parties being opposed to each other, providing voters with only 2 real choices. If you don't agree with your party's stance on every issue, then you inevitably end up voting for someone who will do things you are opposed to.