It was neither High Fidelity nor Merlin Mann that inspired the following very important list of why lists are very important, but the surfeit of posts on <a href="http://digg.com/". title="Digg.com">Digg, Tailrank, and Reddit that are merely compilations of things already known—and, specifically, that these posts are wildly popular. Here's why.

1. Numbers are tangible. The primitive brain that you pretend to have evolved beyond still affects a lot of your decision-making. Marketers know this and exploit it. There is an animalistic comfort in the easiness of lists; they suggest objects to be picked up, grasped, and easily compared. Whether it's horsepower and miles per gallon or gigahertz and megabytes, one number is obviously higher or lower than another. We can rest assured that no one is out to fool us with complex arguments, because the values of the numbers are irrefutable.
2. There is an expectation of legitimate content. With so many posts that are merely ramblings on about one's day, how work is going, or particular political preferences, there is a suggestion that a list of specific items contains information worth arranging. A headline like "Anarchism is Awesome" could be emotional reactions from a 13-year old reading Emma Goldman for the first time is, but "7 Awesome Moments in Anarchism" presents an expectation that anarchism may actually be talked about in discrete and concrete ways.
3. Scannability. Unlike books that require "reading", an archaic and inefficient tradition still practiced by some Luddites and most of the Midwest, on the internet we scan pages by moving our eyes very quickly near the words. All kidding aside, it's true that we really don't read on the web, our eye-pattern tends to look like a capital "F". We jump from item to item, treating paragraphs as lists of ideas, not a slowly unfolding argument. In this way, lists align with our natural tendency, a recipe for success.
4. Lists are easy to remember. Moses knew this when he started calling the rules God unloaded unto him the "Ten Commandments". His contemporary Buddha did the same, whittling life down to Four Noble Truths, an Eightfold Path, etc. Both did so in order for their followers to remember the teachings without having a Treo handy. The number of items in a list present a target of things to remember (how many food groups?) and can remind you of what you're missing. No one gives up after 7 reindeer, because they're obviously forgetting Blitzen. Bonus points if your list can be converted to an acronym, like HOMES for the Great Lakes or ABMNNNNNOPQSY for the Canadian Provinces and Territories.
5. What's number five going to be? Will it be what you longingly hope or what you secretly dread? Lists are dramatic by nature. They have a beginning, a middle, and an end—just like a story. Your brain magically, and mostly subconsciously, transforms the slowly marching items into exposition, rising action, climax, and denouement. As you read through the 13 horror movies, your brain is thinking, "Will 'Jacob's Ladder' be on it?" or, perusing the 10 best Mac apps, you desperately wish to resolve the question, "Has this guy heard of Quicksilver?" By the end of the list, you are rewarded. Countdowns in particular leverage this effect.