Garrett Jones has some good insights into why the Electoral College matters.
We rarely hear too much about regional issues in the U.S. other than farmers vs. everyone else. But if the presidency was decided by majority rule, I'm sure we'd hear a lot more about regional differences. Could a presidential candidate get 75% of the votes in Texas, Louisiana, Mississippi, and Florida by promising broad-based Gulf Coast subsidies and a few other goodies? Could a candidate get 85% of California's and New York's votes partly by offering housing subsidies for people facing high housing costs?
I don't know: But if we got rid of the electoral college and had a popularly elected president we'd sure have a chance to find out.
As it stands, presidential candidates are trying to appeal to the median voter in each state across a large number of states. That's how you get to be president. This reduces regional tensions because candidates are never trying to get 90% of the votes in a state. When you're pitting 90% of one region of the country against 90% of another region of the country, you're substantially raising the probability of social conflict.