One of the problems you will encounter is to decide Nearest Neighbours in non-linear or non-ordered attributes. I'm building on Manuel's entry here.
One problem you will have is to decide on proximity of (1) Seagate 500Go, (2) Seagate Hard Drive 120Go for laptop, and (3) Seagate FreeAgent Desk 500GB External Hard Drive Silver 7200RPM USB2.0 Retail:
Is 1 closer to 2 or to 3? Do the differences justify different categories?
A human person would say that 3 is between 1 and 2, as an external HD can be used on both kind of machines. Which means that if somebody searches for a HD for his desktop, and broadens the scope of selection to include alternatives, external HDs will be shown too, but not laptop HDs. Probably, SSDs, USB memory sticks, CD/DVD drives will even show up before laptop drives, enlarging the scope.
Possible solution:
Present users with pairs of attributes and let them weight proximity. Give them a scale to tell you how close together certain attributes are. Broadening the scope of a selection will then use this scale as a distance function on this attribute.
