There are many map features that do not have an associated rendering. Presumably, this is work in progress? How can one contribute to help clear this backlog?
OSM allows all types of data to be stored. The data then be displayed with different renderers. The renderer that most people initial see is mapnik. However, there are others around which concentrate on differnent features (eg. I can across a public transport renderer the other day that highlights bus stop but downplays other features). Not everything makes it to a particular map otherwise the maps might become too cluttered.
There may be a second reasons for something not being rendered. OSM allows nodes to be given any tag - there is no limit on the key or its value. Thus something might in the database but doesn’t show on the maps because the rendered doesn’t recognise the tag. The wiki contains information on things like approved tags and proposed tags. It might be that the features you are looking at don’t follow a recognised tagging structure.
If you want to help with programming there are some links on the wiki homepage.
Generally the community-driven Osmarender layer (click ‘+’ at the top right) shows a wider range of features than the more classically-styled default Mapnik layer. You can join the tilesathome mailing list to find out more about contributing to Osmarender.