Tuftes basic principle is to use as little "ink" to show let the data provide as much information as possible. I would claim this is still his most important and fundamental principle.

With ink he normally means other elements (line divider, borders, gradients) than the data elements themselves.

Tufte is known for taking any kind of graph or visualization and trying to remove as many things from it and through the process make things clearer than they were.

In this essay he is looking at lists and going through different ways to think about lists and how they sometimes hide important information because of the way they are constructed. He is a known critic of PowerPoints bulleted lists.

Personally I belive he has several great points about information and have many great examples of his work.

But his basic theory I would claim is whats important to practice and once you understand that it really is just a question of practice to apply the principles.