This is so useful, it needs to be at the top. If you are having unexpected behavior, wondering why a particular key works the way it does, use: :map keycombo to see what the key is mapped to. For bonus points, you can see where the mapping was set by using :verbose map [keycombo]. If you omit the key combo, you'll get a list of all the maps. You can do the same thing with nmap, imap, vmap, etc.
Q: NERDTree project explorer doesn't persist on screen after the first :e, must explicitly turn it on with cmd+N. Which setting could make it stay in place by default?