Workshop – Prioritization exercises

I'm planning a workshop in which participants will need to come to a consensus on the merits of a number of product features (there are around 70 features), they will score each feature against business impact and desirability as a group.

The features will then be plotted on a matrix.

I intend to timebox the individual feature discussion -- as there is a lot to get through.

Are there any exercises or techniques I can use to help resolve the inevitable differences of opinion?