Yes doubt, more than three attributes make up effective icon design, but the three elaborated below are a good place to start. For the sake of relative brevity, we have focused on what we consider to be the three main attributes.
Icons that are well designed exhibit a methodical and deliberate approach to the three major attributes that make up any icon design: form, aesthetic unity and recognition. When designing a new icon set, consider each of these attributes in an iterative approach, starting with the general (form) and proceeding to the specific (recognizability). Even if you’re creating a single icon, these three attributes are still implied and can be extrapolated from a single design.