Supporting Standard Targets and Variables
In addition to those I've already mentioned, the GNU Coding Standards lists some important targets and variables that you should support in your projects—mainly because your users will expect support for them.
Some of the chapters in the GCS document should be taken with a grain of salt (unless you're actually working on a GNU-sponsored project). For example, you probably won't care much about the C source code formatting suggestions in Chapter 5. Your users certainly won't care, so you can use whatever source code formatting style you wish.
That's not to say that all of Chapter 5 is worthless to non-GNU open source projects. The "Portability between System Types" and "Portability between CPUs" subsections, ...