I see a great number of sites where the meta description tags are either copied from page to page (i.e. non-unique) or contain only the first 2-3 sentences of the page's content. The former's issue is obvious, while the latter is unwise because the search engines will show whatever content is most relevant to the user's actual query if you provide no meta description, and thus you'll almost certainly get more long tail search clicks by letting the engines supply your description. The exception is if your intro sentences are excellent descriptors of the content on the page, which is sometimes the case with certain article sites or blogs.
I get the whole long tail thing, but overall this seems to contradict what I've come to understand about writing a good meta description, that being to generally write something that concisely and compellingly describes the page's content.
Any takers on this one?