If Obama is what leftist means in the USA (I still don't believe this), then what would you call a communist? I don't know if there is anywhere else where the political compass is so skewed. To most people I talked to when the US elections were going on, Obama is considered the left of the right.

Obama is not a communist by any measure, but he is certainly leftist by any definition - he is for big government, strong unions, strong government intervention in economy via both subsidies and taxation, pervasive regulation of businesses, highly progressive income taxation, abundant welfare services, nationalized healthcare, redistribution of wealth, identity politics. All are common characteristic of modern left.

Could you name one mainstream left concept that Obama opposes?

Obama wants to increase the share of the economy taken up by the federal government. I consider that to be dragging America further to the left, by any standard.

He also appoints people who are activist regulators and judges who view the Constitution as no limit to progressive doctrine.

It might be indicative, or it might not.

Plurality voting systems lead to "least worst" strategies dominating.

They're capitalists voting for a managerial conservative capitalist party. America's "left-wing" Democrats would be right-wing Christian Democrats or right-wing Liberals in the entire rest of the First World and large portions of the developing world, too.

Does is matter what the rest of the world would call it? No. In America, Obama is certainly to the left.