It is not a question of whether the coordinates are an error or not. It is a matter of copyright. See this page: osm.wiki/Copyright
Specifically "Do not use data from copyrighted maps or any other proprietary data!"

It is not clear whether those coordinates from Wikipedia are from copyrighted maps (eg Google Maps) or not. So please don't add them to OpenStreetMap.

Many of the coordinates in Wikipedia are actually copied from other sources, which may be copyright. A lot has been copied from Google Maps, or from various national mapping agencies (eg the Ordnance Survey in Great Britain).
So they are not actually under a suitable licence for copying/importing to OpenStreetMap.

See this page: osm.wiki/Collaboration_with_Wikipedia#Importing_geodata_from_Wikipedia

I agree that the Tracks/Mapped/Labelled columns are not really very useful.

Though I think a single column for completeness is a bit simplistic. Some people are setting it to 100% just because the length of ways in the route relation is equal to the 'official' distance. Without actually walking the route to check whether it is actually correct.

Also, even if the route is correct, is it all tagged correctly? ie I've noticed quite a few parts of routes tagged as highway=track, when they actually paths or minor roads etc. And then there's tags for access, surface, tracktype etc.
Plus facilities along the route, eg toilets, drinking water, shops, cafes, campsites etc.

So I don't think a route is really "complete" unless most of this has been done.

For Garmin maps, I recommend this website: http://garmin.openstreetmap.nl/
Pick the country from the drop down lists, then enter your email address and click build my map.
You will soon get sent a link to download the map. This includes a version with a MapSource installer (called osm_routable_mapsource.exe), so download and run that, and the maps will appear in MapSource. You can then load the maps onto your Garmin GPS device.

Maybe the GPStogo scheme could help, by loaning some GPS devices? http://www.osmfoundation.org/wiki/GPStogo

See this page for a recommended procedure for Naptan bus stops: osm.wiki/NaPTAN/Surveying_and_Merging_NaPTAN_and_OSM_data

Also, there is a JOSM style which can highlight Naptan bus stops, and whether they have been verified (similar to that Novam tool). See http://josm.openstreetmap.de/wiki/Styles

You could be looking at a cached version of the tiles. Try a Shift+Refresh in your browser.

Also, the Mapnik tile server was down for maintenance at the weekend. So it may a bit lagged behind, and serving old version of the tiles.

You could try looking at another OSM map, eg Osmarender (click on the blue + in the top right on openstreetmap.org) or Open Mapquest (go to http://open.mapquest.com/ ).

Is Mainhausen actually a village? Or is it an administrative/municipal area?
If its just an administrative area, then it is already mapped with a admin boundary relation, so the place=village node can be deleted. It looks like that node is from some sort of "openGeoDB" import, I don't know how accurate that is?

See the Wikipedia article: http://en.wikipedia.org/wiki/Mainhausen
It describes Mainhausen as a "community", I don't know if that is equivalent to a village in Germany?

OSM Inspector is updated daily. It says underneath the map when the data is from. It currently says "Data from 2011-08-15 19:59 (UTC)".

For Keep right, I think it is usually every week or two. Though it someimes seems to be less often. It has the date of last update in the bottom left of the page. Currently it says "Site updated at 2011-08-06".

You could try using Yet another validation tool: http://beta.letuffe.org/
It has an option for a Nonames layer, and it seems to be updated fairly often. See for more details: osm.wiki/Yet_another_validation_tool_for_osm_data

"20 years to complete the map" is nonsense. If there aren't many contributors in a particular area, maybe you could spread the word about OSM, and encourage others to contribute. Or you could go and visit those places yourself. And then you will get much more detail than you can by tracing.

If you want a "relatively complete" (low quality) data set, you can just use OS Opendata. There's not much point in dumping all of that into OSM (introducing more errors in the process).
Its much more useful if OSM is a different data set, then they can be compared to find possible errors.

Hawkeye: I don't think adding not:name tags without surveying/checking them is useful. You don't know whether its a mistake in the original survey, or if its a valid alternative name for that street. I think its best to have these highlighted by comparison tools, as places that need checked.

Assuming you are referring to Potlatch (what you get when you click on "Edit"), I don't think saving again will overwrite previous saved changes. Your previous edits should still be there.

Note that the main map (what you get when you click on "View") can take a while to update. Usually it updates within a few minutes or hours, but sometimes longer. So your edits may not show up on the map straight away. Its also possible you are looking at a cached copy of the map, you can try a Shift+Refresh in your browser.

For tagging, maybe you could use the tag for diet: osm.wiki/Key:diet
ie something like diet:halal=yes (if it sells some Halal food) or diet:halal=only (if it only sells Halal food).

Plus the usual tag for the shop type, eg shop=convenience, shop=deli, shop=supermarket etc.

See osm.wiki/Relations/Relations_are_not_Categories
for whether its helpful to have a relation for "all of the tramlines in Stuttgart".

Just start JOSM then do File->Open and pick your JPEG files. Or you can drag and drop the photos onto the JOSM window.
Then (assuming the photos have EXIF location data) JOSM will show a little camera icon for each photo.

Yes, +1 to what Andy says.
Though it would be useful to tag that the track is only usable/visible some of the year. There is a tag for seasonal=yes, but that's not very useful as it doesn't specify which season the track is usable in. Maybe tag it as seasonal:winter=no ?

There was recently an import of loads of airports/airstrips in New Zealand. I don't know how accurate this import is, and whether all of those airports actually exist?

Them only rendering at zoom 10 is a bug. One of the Mapnik maintainers has said he's aware of it, though it should probably be reported on Trac: http://lists.openstreetmap.org/pipermail/talk/2011-June/059140.html

Assuming you are referring to the default 'Mapnik' map on openstreetmap.org, then this is specifed in the osm.xml file. You can get this file here: http://svn.openstreetmap.org/applications/rendering/mapnik/osm.xml
Then look through it to find the colour codes for the tags for each road. They are given as hexadecimal values, but you can convert these to RGB if you want.

Alternatively, an easier option (if you are running Windows): get Pixie from http://www.nattyware.com/pixie.php and run it.
It will show what colour is under the cursor, so if you point your mouse at an OSM map it will give the colours for that.

In UK terms:
If its a public road, that anyone is allowed to drive a car etc along, and maintained by the local council (and usually paved), then its probably highway=unclassified (or highway=tertiary/secondary/primary etc).

If its just for access to a single house or business / car park etc, (so you may be allowed to drive along there if visiting that house/shop, but not have a right to), then highway=service (plus service=driveway for driveways).

If it is for access to fields/forests, so usually unpaved, and the public are not allowed to drive there, then its probably highway=track.

If it is too narrow to drive a car etc along, then its probably highway=path (or highway=footway/cycleway/bridleway).

For rights of way in England and Wales (public footpaths, bridleways, byways etc), they can be tagged with designation (as well as a highway tag): osm.wiki/Key:designation

The tag highway=byway is deprecated. Byways should be tagged as highway=track / highway=path as appropriate, plus designation=byway_open_to_all_traffic or designation=restricted_byway.

If you want a change to Mapnik (or Osmarender or OpenCycleMap), you can add a ticket on Trac: http://trac.openstreetmap.org/
Though it won't necessarily be added - Mapnik is supposed to be a nice looking map, not just a map that shows everything. Is tourism=artwork shown on Osmarender? It usually shows more features than Mapnik.

If you want to make your own map of artwork, you could use overlays. Ie use OpenLayers to show POI on top of a standard OSM map. This would probably be easier than rendering your own map. Plus it means you can click the icons for more info.
See for example London Elephant Map http://www.livingwithdragons.com/maps/elephants/
or a charity shop map I have been playing with: http://green.osmalba.org/charity.html