In the US minority parties get seats on committies and things like that and have some influence there. In a parliment the only power they have is the speak in the parliment how evil the ruling party is.
Actually, the big difference is locational represenatation. In the US, the member represents a specific location, and has to vote for what they wnat (or be voted out). This makes them:
1: responsible to someone other than the party
2: Needing to work together with their region on regional interests (look at all of the NY representitives, both R & D, push for Sandy relief).
3: More willing to compromise, as the “party” will not remove them (although this is beginning to happen as well, especially on the republican side. Joe L. proved to the Democrats that it doesn’t really work).
4: Looking out for themselves instead of the party. This allows them to “cross party lines” when they feel it to be the right thing for their district.