I just got a PM with this question: "What would be the minimum intellectual investment necessary to be able to fruitfully take part in the discussion of decision theory on LW?" This is not the first time I've been asked that. Our new discussion section looks like the perfect place to post my answer:
1) Learn enough game theory to correctly find Nash equilibria in 2x2 games all by yourself.
2) Learn enough probability theory to correctly solve Monty Hall, Monty Fall, Monty Crawl all by yourself.
3) Learn enough programming to write a working quine (in any language of your choice) all by yourself.
4) Learn enough logic to correctly solve the closing puzzle from Eliezer's cartoon guide.
Then you're all set. Should take you a few days if you've studied math before, a few weeks if you haven't. No special texts needed beyond Wikipedia and Google.
A list capturing all background knowledge you might ever need for LW.
(Updated: 2010-10-08)
This list assumes a previous level of education above elementary schooling but less than secondary school. If you start with Khan Academy followed by BetterExplained then with the help of Google and Wikipedia you should be able to reach a level of education that allows you to start reading the LessWrong Sequences.
Nevertheless, before you start off you should read and memorize the Twelve Virtues of Rationality. Not only is scholarship just one virtue but you'll also be given a list of important fields of knowledge that anyone who takes LessWrong seriously should study:
Mathematics:
Basics
Logic
Game Theory
Foundations
Programming:
Programming knowledge is not mandatory for LessWrong but you should however be able to interpret the most basic pseudo code as you will come across various snippets of code in discussions and top-level posts outside of the main sequences.
Python
Haskell
General
Computer sciences (General Introduction):
One of the fundamental premises on LessWrong is that a universal computing device can simulate every physical process and that we therefore should be able to reverse engineer the human brain as it is fundamentally computable. That is, intelligence and consciousness are substrate independent.
Machine Learning:
Not essential but an valuable addition for anyone who's more than superficially interested in AI and machine learning.
Miscellaneous:
Not essential but a good preliminary to reading LessWrong and in some cases mandatory to be able to make valuable contributions in the comments. Many of the concepts in the following works are often mentioned on LessWrong or the subject of frequent discussions.
Keywords:
Concepts and other fields of knowledge you should at least have a rough grasp of to be able to follow subsequent discussions in the comments on LessWrong.
The Quantum Physics Sequence
Complex Numbers:
Complex Numbers @ Khan Academy:
Note: This list is a work in progress. I will try to constantly update and refine it.
A disclaimer on Wolfram's A New Kind of Science: quite a few of the scientists who reviewed it weren't particularly enthusiastic. See for example Cosma Shalizi's review (of special interest to Less Wrong readers, perhaps, for the side comment on Jaynes towards the end! Edit: or maybe not; Shalizi's linked arXiv paper is probably wrong as p4wnc6 explains below). This webpage collects a lot of other reviews of the book as well.