I would say some important parts of a portfolio are:
Some kind of red thread throughout your work. You want to see that it's the same hand that's made the different artwork. You also don't want to aim for quantity instead of quality. A bad drawing can drag down your better work.
Contact info is important of course, like the other anon said. Titles are important as well, and measures if you work traditionally.
An artist statement (or some kind of text) can be really rewarding too. If someone looks at your work, likes it, and want to know more about your work and thoughts, an artist statement is ideal. There's plenty of guides online on how to write a statement.