You don't change the world with a marginally better mousetrap
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Facebook is a marginally better mousetrap, Wordpress and Movable Type are marginally better mousetraps...They just happened to enter markets where the other options were not the greatest.
Marginally better can also be accomplished by simply removing a feature or two... 37signals, Twitter
In an arena without a need for a better mousetrap though you are going up against already established competition, because they are already "good enough". People rarely go looking for better when they have good enough. :)