Well crafted web pages or great features in an app will be undermined if the micro content associated with it isn't right. The little snippets of teaser text, form labels and help text give users confidence and encourage deeper engagement.
I think micro content is one of the weakest points of our website. Why? Because it's difficult. Clear, concise communication is difficult. Reduce this to a small number of characters (like 140 for Twitter) and the job gets harder.
Getting it right takes time and attention. Draft, test or monitor, compare, refine...
But the rewards make the effort worthwhile. Website visitors navigation improved, increases in calls to action undertaken, reduced support calls and queries...
Some great examples of when a little microcopy has had a big impact on the effectiveness of an interface: Writing microcopy - article on bokardo.com
Tips on writing interface microcopy - going beyond help text: Micro Copy: Content Strategy and Writing the User Interface - article on Contini.com by Amy Thibodeau
Going a little broader, Georgie Cohen again talks about interfaces but also talks about the importance of microcontent in social media and 404 error pages amongst others, including some good examples: It’s the Little Things: Why Microcopy Matters - article by Georgy Cohen for meetcontent.com
Related post: Of course Jakob Nielsen has been saying this for years. In this post I highlight a couple of his articles that talk about the impact of microcontent on headings, links and search engine results pages: Search, and the importance of good summaries (November 2009)