we've never had a world leader before, the concept is cartoonish

Maybe people are reading more into the term of World leader than I ever intended.  In my use/meaning of the term, Gandhi, Roosevelt, Stalin, Hitler, Churchill, Kennedy, Johnson, Reagan, Gorbachev, Mandela etc. were all world leaders in the sense of having a profound influence far beyond the formal borders of their own nation state.  Gandhi wasn't even a state leader, and the others had no formal responsibilities beyond their own constitutions/electorates/parties.  The term is neither necessarily good or bad (as the above list confirms) and is usually a mixture of both.

When I use the term in relation to Obama it expresses a hope that his influence will result in the ending of some wars, the development of more effective means of international cooperation and conflict resolution, the emergence of more effective global governance structures to deal will global issues - climate change, financial systems, genocide, famine and disease etc.  

It doesn't mean that I expect him to become some kind of formal or informal world President.  Just that I hope he will be  very good US President who will benefit both the USA and the world.  This doesn't have to be a zero sum game. In GWB's case it was a zero minus game where both the USA and the World were diminished.

Vote McCain for war without gain