Maybe what app stores should do is give apps a 3-star "implicit" review by default, when the user downloads and installs the app. This could possibly also be deleted when the user uninstalls the app. This way, users who don't feel strongly enough about the app either way to give an "explicit" review are still represented in the app's overall rating, and the impact of early good/bad reviews is diminished.

On the other hand, this would probably cause app ratings to tend much more strongly towards the median, which could make them less effective signals of quality.