Version: unspecified (using KDE 4.6.1)
For OpenStreetMap, Marble downloads raster images of the maps that are offered by the OSM server (probably those that are closest to the level of zoom of Marble), and then has to resize them to fit the actual zoom level. This means that the map image is blurred and ugly and in the latest version the readability can also suffer for some zoom levels.
The maps provided by the OSM server are rendered wonderfully by Mapnik, but the postprocessing done by Marble destroys them.
This is true for all raster map sources, such as OpenStreetMap and Osmarender. It doesn't affect maps such as the atlas, the natural earth one, or the maps of Mars, Venus and the Moon.
It would be very nice if Marble could provide exact zooms for those maps (perhaps through a new projection mode?) so that they are not resized, or could render the maps itself with the right projection, or could otherwise make them sharp and comparable to those on OSM's website.
*** This bug has been marked as a duplicate of bug 199259 ***