There are two main types of passive investing, asset class investing and indexing.  Indexing is a great alternative to investors without access to asset class mutual funds.  However, if given a choice, asset class investing is superior. 

Indexing relies on commercial benchmarks to define the investment strategies.  On the other hand, asset class investing involves the application of academic principles.  In other words, academics have defined the major asset classes, such as large growth and value, small growth and value, international large growth and value, and international small growth and value, with a rigorous screening process.  And instead of investing in a sampling of these asset classes such as the Barra Large Value for large cap value exposure, you would instead invest in every stock classified as large value by an academic definition.  Also, for asset classes like large value and small value, there is no such index that can capture the premiums you should expect by investing in pure value and pure small companies as defined by academia.