In general, I'd say you're right that the fact that "some one else already does it" shouldn't stop you from taking on a project. Almost every successful site on the Net was NOT the first to do what they do (Google, Facebook, Basecamp, MailChimp, SurveyMonkey, Woofoo, WordPress, etc.) yet they're all pretty amazing. The largest factor of your success/failure will probably be your determination, so just go ahead and pick the thing you'll be most likely to love doing for the next X years.
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In general, I'd say you're right that the fact that "some one else already does it" shouldn't stop you from taking on a project. Almost every successful site on the Net was NOT the first to do what they do (Google, Facebook, Basecamp, MailChimp, SurveyMonkey, Woofoo, WordPress, etc.) yet they're all pretty amazing. The largest factor of your success/failure will probably be your determination, so just go ahead and pick the thing you'll be most likely to love doing for the next X years.