There's a whole internet-savvy generation that uses the High Street as an opportunity to examine and test goods before returning home to find the best deal in the internet. The poor retailer maintaining expensive frontage and high staffing is increasingly doing so for the benefit of the manufacturers of the goods he stocks. This doesn't apply to time-limited or quality-critical goods such as cheese, fresh meat or flowers as much as for white goods and electronics, but nevertheless the 'imperfect knowledge' that previously gave the High Street a living is fast disappearing. If local retailers can't compete on price, what else can give them competitive advantage?
For the vendors of cheese, fresh meat and flowers the killer is not price but parking. Rapacious councils desperate to rake in every penny are killing their own High Streets with sky high parking charges and the rationing of parking opportunities. Again, what can such retailers do?
This is a hard one for economic libertarians who would generally rather the State didn't intervene to distort markets, but in the case of High Street parking charges one could argue they already were. Also in the case of a planning system that favours the giant corporations (whatever their complaints).
I'm not sure that Mary Portas has the right answers, but it's the start of a useful debate.