Currently we only have wgVersion but many changes occur on live wikis between version.
It may be possible to have some kind of feature array but that could be bulky.
Best might be a universal wgApiRevision which is not dependent on svn and thus will be the same even if you're on a forked wiki, forked api.php would be another story however.
In any case it's surely a good idea to expose the information provided by api.php's "version" parameter. Currently it only outputs flat text when viewing help but could output structured data for JSON and all formats.
In my case I have three ways to achieve my goal. If api.php is not available I can do several ajax hits to the server which results in partial information. If api.php is available I can do a single ajax server hit but need to request more data than strictly necessary. If api.php supports the "amlang" parameter for "meta=allmessages" I can get everything I need with a single efficient ajax server hit.
Currently, however, there is no deterministic way to know if "amlang" is supported.