I know I overuse that and it. That (Ha-Ha :-) is because I am so afraid of overusing keywords. I wrote an article on an editing software and wondered about 'Article' as keyword and 'online article' as a phrase. So, I used it and that to stay away from using those. That is probably due to my inexperience and understanding what keywords are and what is stuffing. Essentially I stayed away from using the formal name of the software realizing that for sure was a keyword. But, wondered about article and edit too. In other words does a noun become a keyword with percentage usage to word count even though not the subject matter?
The problem with the overuse of "that" is the fact that we tend to write the way we talk. Too many "that"s slows down the reader. Also, if you are referring to a person, you should use "who" instead of "that". People are "who"s. Things are "that"s.