Outside view on USA politics.
Democrats: Owned by corporations. They pass laws to help the corporations.
Republicans: Owned by corporations. They pass laws to help the corporations.
Third parties: Might change things, but the population seems happy enough with their corporate sponsored two party system.
Agreed. Here in the UK we don't get loads of news about the US elections until they happen, but I have seen that both parties are constantly spending money on campaigning with that money coming from corporations and generally making a lot of noise without much backup. I've also seen the Republican candidates seem to be fairly crazy and out of touch: 999 tax plan, moon mining bases, joking about $10k bets, forgetting what their policies are.
Whoever gets elected next year is going to have a tough four years. America seems to be waking up to the fact it isn't the world's greatest super power any more and I doubt it's going to go down well when it's bundled together with debt, the style of public politics over there and general anger about living standards decreasing/not improving greatly for most people.
With regards to polar opposites, the UK seems to go in cycles. Our parties started on the left and right, came into the middle and they're now going back again. Some people might say it's because of how the funding system works here (a small few giving a lot of money to the Conservatives, trade unions giving money to Labour, not many funding the Liberal Democrats). As the for the US, from the outside it seems that the Democrats are just right-of-centre with the Republicans being right (some being far right). Not having lived there myself, I'm not sure how accurate that is as that would imply that there's very little appetite for leftist polices.
I guess if people are pissed off enough, they will actually vote for a third party.
UK's Liberal Democrats. 3rd party, now part of the government at the last election. But now they're really far behind in the polls. However you would be surprised how many people still think otherwise in a First Past the Post system: "why should I vote for [3rd party]? They have no chance of getting into government, so it's a wasted vote". Somewhere with a proportional system (most of Europe), you'll find there isn't a "3rd party" in the sense of "useless party".