How could this work? Well my company is trying something like that now, though it is early going. The easiest way to do this is by filtering the Web by domains instead of individual web pages. This alone removes several magnitudes of complexity.
many topics tend to bleed across niches
sites themselves will change purposes over time
how sites are monetized (and other user experience choices) may change over time
people buy and sell sites
sometimes the desired information is only accessible on sites which are not particularly popular because they are not aligned with moneyed interests
sometimes sites that are popular might be popular because they are inaccurate, conforming to an expected + desired bias
if the whitelist is black and white, entities may change their approach after being well trusted (one of the elegant aspects of Panda is how it can remeasure over time)
One idea would be to have sites that concentrate on some topic(s) of interest, where people can submit interesting links they find directly, and where others who share those common interests can lend their support to promote the best material and share other related links they've found themselves. I know, I know, it's a radical concept, but I truly believe there's some potential there...
Then you could maybe even organize them into publicly based on those tags. You would need a hierarchy of tags then though but that would give you a rather comprehensive, yet still curated, view of the web...