Yeah, you're supposed to elect the other major party. Who did exactly the same the last time they got elected.
One party wants to hike taxes on the rich. The other party wants to cut taxes on the rich.
One party has a pretty extensive health care reform plan. The other party would repeal that plan if possible.
One party wants to spend more on the military. The other party is trying to end wars.
One party keeps abortion legal. The other party wants to end that.
One party favors economic stimulus. One party opposes it.
One party favors using the fed to achieve full employment. One party believes in opportunistic deflation.
One party wants to invest in infrastructure. One party doesn't.
One party wants the federal government to support the states. One party wants the states to play a larger role.
One party favors an assault weapons ban and streamlining gun regulation. One party believes this is an infringement of liberty.
One party started a mortgage modification plan a couple years back and the other party launched a massive national 'tea party' to oppose it.
One party believes in fighting climate change. The other party fillibusters all such efforts.
One party believes in regulations on banks. The other party doesn't.
One party is in favor of abstinence only education and intelligent design. The other party thinks those are stupid.
The last time they were in power one party cut taxes overwhelmingly for the rich, started two wars, gutted the social safety net, cut back on a bunch of regulations and passed a law banning federal recognition of same sex marriage.
The last time the other party was in power they hiked taxes on the rich, dramatically cut back on the number of soldiers overseas, created a new healthcare system for children (SCHIP) and a subsidized healthcare market for the poor, started regulating greenhouse gasses and passed laws allowing gays to serve openly in the military and protecting them from hate crimes.
...so yeah, other then then those differences and about a million other differences I could list if I had the time and inclination, there isn't much difference between the two parties.
I favor a proportional vote system as much as the next guy. I would love to be able to favor a candidate who more closely matches my views. But people saying that the two parties are identical is a HUGE part of why we have so many problems. Anyone remember how everyone said there was no difference between Gore and Bush? Furthermore, a lot of the crap that people blame the democrats for not doing they didn't do because the republicans fillibustered it despite it having a majority vote. Don't blame the democrats for not having a time machine and going back to change our political institutions.