More Inside, or [more inside] as it is formatted on MetaFilter, is a feature that provides the option to expand a post when first publishing it to the site, instead of elaborating in the first (few) comments, as it used to be done. This feature was first added to AskMetafilter circa 2006 or earlier, then formally added to MetaFilter, MetaTalk and Music on September 6, 2007, long after the portion of this page from [more inside] is your friend down was originally written (Dhartung wrote it on December 13, 2002).
The portion below is kept for it's place in history, and because brevity is still enjoyed, though longer posts are also valued. In fact, MegaPosts have become A Thing, called megalink posts in the Best Post Contests.
And if you're interested in numbers, pb has them for you, with 36% of posts between September 6, 2007 and January 6, 2012 including More Inside.
[more inside] is your friend
In general, a Post should be as short as possible. Sometimes, to introduce a topic, you may want to add a large number of supplementary links, an extended quote, or just ramble on at length to frame the topic the way you would like. (The first two are good. But if you find yourself going more than a paragraph, you should really reconsider the brevity of your post as a whole.) Long posts are unpopular, unless they are exceptionally worthwhile in their length. They take up a lot of screen real estate on the front page, and they can force someone to scroll. Multiple paragraphs can resemble multiple posts, which is confusing to readers scanning the front page.
If your post runs to more than one paragraph, or more than five to seven lines, consider moving most of it to a comment inside the thread. This is signalled by ending your brief Front Page Post with the text [more inside]. Note that sometimes Matt Haughey will do this anyway -- so why not save him the work?
Note: Do not abbreviate to [MI] or [+], as has recently become popular on AskMetaFilter. The use of [MI] or [+] is incredibly dumb and annoying, a sure sign of inconsiderate laziness. The community doesn't like it. Compare FPP.
Posting a [More Inside]
The next problem is that all the text you have from that overlong post will now be in a comment -- but you can't write the comment until you preview and post your Front Page Post!
A [more inside] implies a presumed right of first comment to the poster. MeFi etiquette suggests that readers seeing this should wait a little while before adding their comments. Too long, though, and somebody will break the taboo. This can lead to situations where you are still formatting your first comment, perhaps even researching URLs to add to it, while people are already responding to your post and putting comments in the thread. They're probably snarking about where the poster has disappeared to, which can spoil the atmosphere of your thread even before it's complete in your mind.
To avoid this, the poster should write the first comment in advance using a text editor, and have it ready before hitting the Post button for the Front Page Post (don't forget the [more inside]!). This way it is easy to get the first comment posted; the Post button will put you on a fresh MetaFilter front page with your post at the top. Hit Post Comment and cut and paste from your text editor, Preview, and you're done.
If you have prepared your entire [more inside] post this way, nobody will step on your toes by commenting before you.