Speaking for myself, two items I often find missing: the overall summary and the rationale.
Many times I've gone to a set of documentation wondering what exactly the thing _is_. Let's take the JavaScript language, for example. The very first thing that should be documented is a short description of what exactly JavaScript _is_. This could be a single sentence, should probably be at least a paragraph, and should be no more than a page. It boggles my mind how often this critical piece of information is omitted entirely.
The rationale quickly orients the reader to the landscape of similar solutions. It's very rare that something has no peer of any kind, and it's quite useful to understand why this particular thing exists, and how it can be distinguished from other similar solutions. The Python language, for example, has a number of attributes that separate it from other computer languages. This can be concrete (it comes with constraints that lend themselves to readable programs), or they can be aspirational (Python aspires to be an easy language to pick up, particularly for non developers). This can go deep, and rapidly descends into matter that my be quite useful and interesting, but belongs in an appendix or other separate matter.
Many times I've gone to a set of documentation wondering what exactly the thing _is_. Let's take the JavaScript language, for example. The very first thing that should be documented is a short description of what exactly JavaScript _is_. This could be a single sentence, should probably be at least a paragraph, and should be no more than a page. It boggles my mind how often this critical piece of information is omitted entirely.
The rationale quickly orients the reader to the landscape of similar solutions. It's very rare that something has no peer of any kind, and it's quite useful to understand why this particular thing exists, and how it can be distinguished from other similar solutions. The Python language, for example, has a number of attributes that separate it from other computer languages. This can be concrete (it comes with constraints that lend themselves to readable programs), or they can be aspirational (Python aspires to be an easy language to pick up, particularly for non developers). This can go deep, and rapidly descends into matter that my be quite useful and interesting, but belongs in an appendix or other separate matter.