I would add sulfur and phosphorous to the list of most important elements, sulfur being in two of the twenty protein amino acids and phosphorous an essential component of DNA (with various other important biological roles). Many metals also play essential roles like iron (present in hemoglobin, cytochrome c, and many other proteins), zinc (often an essential part of DNA-binding proteins), cobalt (in the vitamin cobalamin, essential for nucleotide synthesis), and others.
Selenium has a biological role and is an important cofactor for some enzymes such as glutathione peroxidase (involved in antioxidant activity). While selenium is toxic in moderate quantities, it is still an essential micronutrient. So, if you completely remove selenium from your diet, you will have some problems. Also remember that anything, even water, is toxic at high enough concentrations.