Description meta tags are important because Google might use them as snippets for your pages. Note that we say "might" because Google may choose to use a relevant section of your page's visible text if it does a good job of matching up with a user's query. Adding description meta tags to each of your pages is always a good practice in case Google cannot find a good selection of text to use in the snippet. The Webmaster Central Blog has informative posts on improving snippets with better description meta tags18 and better snippets for your users19. We also have a handy Help Center article on how to create good titles and snippets20.

Let me tell you a story. Early in my tenure at Yahoo we tried to get into the site dev process in the early stages in order to work SEO into the Product Recommendations Documents (PRD) before wireframing began. But as a fairly new horizontal group not reporting into any of the products, this was often difficult. Nay, damn near impossible. So usually we made friends with the product teams and got in where we could.
Having a different description meta tag for each page helps both users and Google, especially in searches where users may bring up multiple pages on your domain (for example, searches using the site: operator). If your site has thousands or even millions of pages, hand-crafting description meta tags probably isn't feasible. In this case, you could automatically generate description meta tags based on each page's content.
Thanks for the great post. I am confused about the #1 idea about wikipedia ded links…it seems like you didn’t finish what you were supposed to do with the link once you found it. You indicated to put the dead link in ahrefs and you found a bunch of links for you to contact…but then what? What do you contact them about and how do you get your page as the link? I’m obviously not getting something 🙁