Google Checkout,

- Not present at all in a number of countries

- Impossible to sell from an even higher number of countries

- No customer support at all

Paypal

+ present virtually everywhere

- customer service generally considered frustrating, becomes terrible when dealing with their fraud detection

iOS AppStore

+ present in more than a hundred countries (though some parts e.g. iAd may not be available everywhere)

± customer service generally regarded as good, except when dealing with the approval process (and even that area shows progress)