Hi Alex,
There are some differences: Suburban stations are located in urban areas, while local stations are located in rural areas. Suburban stations typically have a higher number of passengers than local stations. The most important difference: A suburban stations is typically served by more trains per time interval and have a higher number of passengers than a local station.
I meant to say there are no differences between these two that are important in terms of station dominance. (Sure there are differences, but they are not relevant for the zoom level display thing we're discussing at the moment, and neither for related things like search result ordering) In other words: I don't see any reason to treat two stations differently only because one is inside town and the other one is not.
Also, we'd need some way to differentiate Basel Bad Bf from Basel SBB, Berlin/München Hbf from Ostbahnhof, København C from Nørreport. In each of these examples, both mentioned stations have international traffic, but one is definitly to be displayed at a lower zoom level than the other.
This problem was also discussed on the wiki discussion page: http://wik i.openstreetmap.org/wiki/User_talk:Rurseekatze/Station_Importance_Draft
I proposed to tag both of those stations with the same category, but use additional information like passenger=* to determine the most important station within one category.
I don't think it's a good idea to rely on data like this which is not always easy to acquire (think of bus stations etc), and is probably not measured the same way in different places etc. I think even without knowing passenger counts we humans know quite well how important stations are relative to another, we just need a good way to store this information.
Marian