The first thing I did not know from the beginning was that keywords are hierarchical. Based on your question, I see you know that so you are already ahead of where I started.

Other than that:

• Make clear a hierarchy that has an unambiguous meaning even at the expense of being redundant. For example, city hall can be anywhere but montreal city hall has to be in Montreal.
• Avoid plurals (I'm guessing you can avoid singulars instead). Consistency is key. Same thing with case.
• Watch out for the auto-complete, it often picks something similar but not exact, plus if you made a typo, then it copies them all over the place.
• Make META keywords with a different notation. Meta keywords are things one would NEVER search for. For example, Location (notice the capital) regroups america, europe, asia etc (notice lower case).
• To keep lists from getting long, subdvide and reorganize often.
• Designate a keyword for images that do not count. While you can look for images without keywords, there needs to be a way to distinguish the images which have not yet been tagged with images that will never be tagged. I solved this by using the source keyword and all my smart collections exclude images with that keyword, because it marks them as intermediate image files (panorama pieces or hdr brackets).
• Tag with the aspect ratio anything remotely standard. Lightroom's Aspect Ratio filter should be called Orientation. This makes it impossible to search by aspect ratio after an image has been cropped and tedious before (since you have to search for all cameras that output at a certain ratio).