The navigational possibilities of labels would increase if one could filter by multiple labels.
For instance, after you choose a label you get a list of pages that have that label. Ideally, if you had the ability to filter by additional labels, you would be able to winnow down that list, by choosing additional labels which are also assigned to these pages. Currently, that is not possible and information stored in those labels cannot assist a user while navigating.
The potential uses of labels (or tags) in a wiki environment have not been fully explored. I think there is room for many innovations with labels. However, the inability to filter by multiple labels is a roadblock.
In del.icio.us you can filter by multiple tags. Go to a user's page and choose one tag. Under the title "related tags" there is a list of tags that are attached to the same pages. You can add another tag to the filter by clicking on the "+" sign to the left of the tag. You can also remove a tag from the filter.