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One feature that I very much like are anecdotes or jokes or the like about the topic at hand, in books as well as in lectures. The best ones make you think about an important topic that most people would miss otherwise, like
John von Neumann to Werner Heisenberg: "QM has lead to beautiful mathematics, but we mathematicians payed back by noticing such important concepts like the difference of selfadjoint and symmetric operators".
Werner Heisenberg: "What is the difference?"
From time to time I'd like to include an anecdote like this on the Wiki, I added one on the page Random number generator. Any opinions? Do you like it?