In terms of customisation, from an online retailing perspective, I don't really see the benefits against cost of development. You could provide a 'Windows Live' type page where users can customise what they see and how they see it, but frankly with mature retailers I don't see who would really use it. Shared personal profiles that users can customise similarly don't really make sense for this type of business, or at least not until social networking in the mature brand shopping context becomes significantly more embedded, if it ever does.
However, simpler customisation linked to personalisation, could be very useful; for e.g. possibly like a variation on the Amazon recommendations model, where the user could be presented with an option alongside product displays allowing them to select items into a 'not interested' and 'I like' category that can then not only automatically build them their own page they can refer to and edit later, but also feeds into the intelligent browsing stuff I was talking about in the previous paragraph - i.e. this information is used to understand what the user is looking for and preferentially feed them those products as they browse or search, thus enhancing the targeted browsing experience.
This could help retailers improve their offering to customers online, increasing not just browse to sale conversions, but also brand loyalty because the customer feels understood.