Currently we have no guidelines for deciding what should be a Property and what should be a Category. These two namespaces appear to have some overlap in the existing OpenEI. As OpenEI has grown there appears to be no clear rules for Properties and Categories.
Let's start drafting guidelines here and iterate until we get something we're all happy with.
Categories should be used for grouping things into "buckets" like Books, Tools, Countries? This describes what the items have in common.
- Categories should be plurals. 
- Example: States or Countries
Properties should be used for specific attributes of an object, e.g. a book has an Author, whose name is Asimov.
- Properties should be verb phrases. 
- Example: Has population or IsPartOf
- Exception: Is a is not a valid verb phrase. (See Below)
Number of Elements Test
The number of elements test, is a decent way to determine whether something should be a Category or a Property.
Categories typically have a two-element relationship. For example, I, Robot is a Book. This can get a little confusing when using the verb guidelines above, unless you adhere to the exception mentioned above. So, ruling out is a as a valid verb phrase, iRobot would fall into the category of Books.
Properties typically have a three-element relationship. For example, I, Robot has an Author whose name is Asimov. Or, I, Robot was first published in its FirstPublicationYear which was 1951. It would not make sense to add I, Robot to the Category of 1951 or Books That Have Been Published.
One might be tempted to add I, Robot to the Category of Asimov Books. However, it's important to note the truncated three-element relationship at play here. What do all these books have in common with Asimov? What is his relationship to all of the books in that category? By contrast, Sci-Fi Books renders a two-element relationship. It helps that Sci-FI is commonly known to be a category. There are no obvious questions about what all the books have in common with 'Sci Fi'.