(Written responding to a friend asking about the party I support)
The fundamental difference between Republicans and Democrats is one: the role that government plays. All the social wedge issues? Just talk.
I believe the government has the ability to solve many of our problems, and I also believe that Democrats over the last 100 years have been more consistent, more fiscally conservative (save FDR), and more in line with principles that I believe.
On the surface the parties do not represent what is core about each group. For example, on the Democrat's side you look at Occupy Wall Street, a sliver of what is representative of Democrats, and paint the whole party in a similar manner. What is interesting about the OWS movement, is the wealthy blue states pay for all the social programs of the red states. Also, of the richest 20 individuals in the US, 12 vote Democrat which is again inconsistent with the OWS sliver.
Let me give you another example to why I side with Democrats over Republicans; the only true conservative in the Republican primaries is Ron Paul. He is small government. He is, by definition, a true conservative. An actual conservative should embrace this guy like their own mother, but they don't. Why? The Republican party has an identity problem. They do not understand the definition of conservatism and wrap themselves in issues like gay marriage and abortion which has as much to do with conservatism as Chuck Norris does to baseball. Republicans vote socially, and instead of wedge issue legislation their leaders grow government, give disproportionate tax decreases which shifts tax burden, and funnel more money into the military and subsidies for industries that fund their elections. Jon Stewart said it best ... "Republicans are for limited government...limited to the stuff they want to do."
Economic policy is another example as to why I side with Democrats. Every single candidate on both sides of the aisle are Keynesian except one (RP). One of the fundamental beliefs of Keynesian is the idea that you can deficit spend (ie use government -- a core Democratic principle) to overcome recession and economic busts. Although Republicans like to pretend they are fiscally conservative, I struggle to find a concrete example of when they demonstrated this principle.
The most popular Republican president in the last 100 years was Reagan. People label Reagan as a true conservative, but he was far from it. He presided over two substantial tax increases because governmental spending was out of control, provided amnesty to three million illegals, had a very liberal foreign policy (which I like), and expanded government spending by 60%. Reagan was respected for the bipartisan deals he made, never went to church, and was very silent about his pro-life stance. Reagan would be crucified by the Tea Party, but today he is shrouded as a true conservative. Why? The Republican party has become the voice of the extreme, forgetting their core beliefs. Compromise is an ugly word and moderates, the same people that gave Reagan an astounding 49 state win in 1984, have switched sides, effectively giving Obama a landslide victory in 2008.
Democrats also have an identity problem but this is due to the wide umbrella the party casts on smaller groups. This is the reason Democrats have 20 million more registered voters than Republicans. I can be a pro-life, anti-gay marriage Democrat and be completely supported by my party. Could you be pro-choice, pro gay-marriage Republican and say the same? Democrats are more accepting of differentiated opinions and positions. There are examples of moderate Democrats, but after the 2010 elections, I struggle to think of any moderate Republicans.
Your question is a good one, and I am answering it because I know its sincere. You look at me and wonder why I support the party I do. I look at you and ask the same question.