OpenStreetMap
An idea that I saw on the mailing list (thank goodness I don't subscribe to that) was that of different layers of data. The idea would be choosing what "layers" of data you want.
Certainly it would make editing a lot easier as some areas are unwieldy to deal with.
Thinking about it, XAPI does (did?) this.
Discussion
Comment from Gnonthgol on 18 January 2012 at 12:35
Editors like JOSM have filters that can make areas less cluttered and easy to work with.
Comment from AndrewBuck on 18 January 2012 at 20:22
The JOSM filters work for data you have already downloaded but don't allow you to choose what you download in the first place. It would be nice if the XAPI servers had functionality that would let you download a subset of the data, I don't think they currently support this.
For example if you ask for the road layer you would get highway=*, cycleway=* and several other things. Asking for the water layer would get you waterway=*, natural=water, landuse=reservior, etc.
This would let people who are working with only one kind of data (perhaps you are cleaning up the lakes/waterways over a large area) and you only need a subsection of the full dataset.
Using the layer download you put less strain on the servers, and equally importantly, your editor doesn't slow way down because you have thousands of nodes for buildings and roads which even if you filter out, still slow down the editor needlessly (assuming you are working on the waterways as given in the above example).
Also, XAPI servers limit your download in two ways, by the size of the area you query and the number of objects in the resultset. By allowing you to download only some subset of the data you may be able to download a larger area since you will not be as likely to hit the object count limit as you are not asking for the portions of the data you don't intend to use.
-Buck
Comment from SuborbitalPigeon on 18 January 2012 at 22:56
I thought of a snag with the layer idea, is that you could forget what you had downloaded and add something that was "filtered out" again.
I hadn't actually realised that JOSM could filter data, but I'll have a look.