Inspired by a part of this question, this seemed like an interesting question to ask. Advocates of LaTeX often say that you can see the difference in typesetting quality when you look at a document prepared with LaTeX and compare it to something prepared in, say, MS Word. I certainly don't dispute this; I can often tell, but it's hard to put my finger on exactly why. So what are these differences? What are the distinctive features that make a document look like it was prepared in LaTeX, rather than some other system? In other words, if I have a PDF file and I would like to make an educated guess as to whether it was created in LaTeX or not, what should I be looking for? (ignoring any PDF metadata that identifies the creator)
I have my own ideas about this but I'll refrain from self-answering, at least for now, to see what other people may suggest.