In the spring of this year I have mapped the vast majority (>95%) of shops and retail areas in Nottingham, the 8th largest retail centre in the UK, serving a market of around 1 million people. In addition I have repeated the exercise for a Maidenhead a town of 70,000 which is only a local shopping centre.
Comprehensive mapping of retail outlets allows a more rigorous analysis of OSM data, with significant implications for how we should collect and map data for shops. These include: better support from mapping tools (particularly for mapping change), defining a usable classification for retail areas, and improving consistency in tagging.
These need to be cross-checked for different countries, so I hope this provokes extensive contributions from other participants.