This question has been asked several times already but I like the way Viqi French covers it. In particular how she compares it both to an art form (think DJ's or Museums - interestingly her post is illustrated by a Basquiat painting) and to publishing a newspaper or a magazine. Obviously that last one is dear to our heart at Scoop.it...
The other aspect I like are the best practices at the end of the article - a curation "101" if you want.
One of the things we’ve been trying to do at Scoop.it is to give a measure to guide your curation effort. This is how we recently introduced the Scoop.it score that measure how well you perform on this best practices (we have more or less these criteria than the one she lists).
We’re just at the beginning of this but we’re the first curation platform to ever do that to my knowledge.