I'm Joachim from Stuttgart and you can contact me via OSM personal messages or on my Wiki user page.
2016-10 Edit of capital=* on Russian county capitals
Motivation and tagging discussion
The tag capital=* has it's place in our tagging scheme. It's currently used by at least osm-carto and hot styles on osm.org. The alternative of using the admin_centre role of boundary relations are not supported by osm2pgsql.
An overpass query on all admin_level=2 borders runs longer than 3 minutes (stopped it at this point) while a overpass on all capital=yes node runs in seconds. Also there is some discussion how to handle use the roles for countries with differing admin centres and capitals.
After a clean-up of capital=yes of around 40 cities the only remaining cities tagged with capital=yes which are no country capital are in Russia. There most capitals of federal subjects are tagged as capital=yes + admin_level=4 and some other with admin_level=3/6 - all in all currently 73 nodes (overpass). The motivation is to get a unified tagging for capital=* - i.e. capital=yes for country capitals and capital=[admin_level] for lower capitals. The current hybrid approach of using capital=yes for all capitals and using admin_level to further specify it leads to semantic problems concerning admin_level=* see a message on talk. It also leads to needless complex SQL-query engineering  with regressions  .
Since talk-ru mailing list is empty I proposed to change all to capital=[admin_level]. This proposal lead to some discussion about tagging for the renderer and the notice that boundary relation and role admin_centre should be preferred. So I changed my proposal to delete the key capital=yes from all cities in Russia except Moscow. In line with the lack of interest for the tag this did not yield more (serious) responses.
I want to remove capital=yes from 73 place nodes in Russia using overpass ( http://overpass-turbo.eu/s/jfZ) and JOSM - Moscow keeps capital=yes. As of 2016-10-13 the tagging has been has been done with changeset (Visualise). The following three cities have also been modified, as they had capital=yes without admin_level: Naltschik, Chabarowsk, Tscheljabinsk.
The long query run time of overpass was because of bbox added by the wizard - this is running very slow for relations. The following query is quite fast:
[out:xml]; ( relation["admin_level"="2"]["type"="boundary"]["boundary"="administrative"]; node(r:"admin_centre"); ); out meta;