The biggest hot button issues are abortion and gay marriage.
Why are social issues more likely to push hot buttons than economic ones? Just about any given economic issue (like taxes, the deficit, business regulations, labor issues, etc.) will have a bigger impact on more people on a day-to-day basis.
I personally think it's probably because economic issues can be talked about scientifically, while social issues are mostly philosophical in nature. It's usually impossible to reconcile differences over social issues for that reason.
Plus, social issues usually have religion mixed in in some shape or form, and - no offense to the religious of the board - religion is one of the biggest things that makes people unable to have a rational discussion with someone who disagrees.
