The classic difference between American and British English is the extra -u- in some British English nouns. Perhaps because it is the most obvious difference between American and British written English, the -our spelling allows some writers of particular national prejudice to maintain a feeling of superiority to their seemingly simpler cousins across the pond. However, the British English convention is nowhere near as consistent as the American (with glamour perhaps our sole exception) and has slowly—ever so slowly—been adopting the more consistent American spellings.
While most agent nouns in British English have lost the -our ending, such as actor and governor, saviour and paviour (a variant of paver) still maintain the British habit. But with non-agent nouns, British English remains inconsistent. For every noun with the -our ending, another similar noun takes the simpler -or rendition: horror, pallor, tremor, error, liquor, stupor, terror, etc. Even some words that do use -our in their root form, such as vapour and colour, lose it with certain suffixes—vaporize and coloration.
If we set aside nationalist fervor for linguistic curiosity, we may see that the -our habit is inconsistent and serves no aural or logical advantage. Although slowly, words historically written with the -our ending are also being replaced with their -or counterparts. British English is changing, but it may take a few influential writers to quicken the adoption of the simpler spellings. For now the two branches of English remain separate on this matter, though American English writers may breathe a sigh of relief to know that their work is made a little easier in this one regard.