More to passive investing that just ER...

[mickeyd]

Pretty good recent Swedroe article on this topic.

For passive investors, the choice is much simpler than it is for active investors, because the universe of funds from which to choose is much smaller. However, even for passive investors, the choice is not as simple as just looking at the expense ratios of the various alternatives and choosing the cheapest alternative. The reason is that not all "index" funds are created equal.

Not All Passively Managed Funds Are Created Equal - Research