The reason why I brought up segmenting players based on difficulty is that higher difficulties are supposed to have a chance to drop better gear. The problem arises when a player in a lesser difficulty is simply able to buy gear that is not normally attainable to them (i.e. only drops on a higher difficulty), which gives him an edge. As much as having an auction house may ruin the spirit of a gear grinding game, the players are still acquiring gear that they could technically farm themselves.
The nature of the shared stash makes me think that players may not be segregated...
There's no segregation based on difficulty level. I have no proof of that specifically, but if you dig around you can find Blizzard quotes (not sure if it was interviews or battle.net posts or both) stating they see nothing wrong with twinking lower level characters, so the shared stash is just one way of promoting something most people want to do anyways. I mean, part of their business model relies on item trading...they're not going to purposefully implement ANY barriers to trading.