After my first two weeks or so of intensive mapping and editing my hometown (Helsinki, Finland), I have some observations.
1. Here's plenty of unconnected ways, mostly cycleways. Not that much but something to check when editing areas.
2. Oneway attributions are quite often missing (in Eira all of them).
3. Some streets have been mapped from aerial images, which have had distortions; for example, Aleksanterinkatu is a straight line but was not such on the map.
4. I have also corrected several cycleways/pedestrian roads which were tagged as residential; routing would be a disaster if these are not fixed as many of them even have bollards to stop motorized vehicles.
5. If trams are tagged on the same ways as the underlaying highway, the tracks don't get rendered in the Mapnik layer, unlike Osmarender.
6. Yet to find a maxspeed-tag anywhere on the map. It would be possible to deduct these from country specific rules, if it weren't for the fact that Helsinki has lowered most speed limits from the 50 km/h in urban regions to 40 and 30. I, too, haven't started tagging them if I don't have any other changes to make.

The first two notions are in my opinion the most important ones: incomplete data leads to inefficient routing, which I believe to be the first major use for this map data.

I just today tried to improve the Länsi-Pasila region - tracked the roads navigable by car and marked the others as service/pedestrian/cycleway, whichever seemed most appropriate. Last edits to those ways had been made with "potlatch alpha" and no tracks existed before so I guess they were quite old.

Also, as some buildings had already been mapped in the city center, I attempted to locate the corners of some other buildings near me. It seems possible, although at some locations with bad reception conditions I have to make several approaches from several directions to get a reliable approximation of the location and stand still for a while at nearby locations with better accuracy to "calibrate" my position. Mostly this happens between high urban buildings. From the track log (once per second) and my memory of the route I walked it's possible to deduct real directions and distances traveled. Luckily Helsinki has adequate aerial images available so that the proportions can be crosschecked with those. Mapping buildings is something I do when I don't have the time or transportation available to go mapping some uncharted territories.