RealMedia mostly depends on getting working codecs. ffmpeg has made some progress recently with the audio codecs, but there are still problems with the demuxer and no available decoders for realvideo (rv30/rv40). This is going to take a long time.
QDM2 was added in 0.8.4, so Quicktime is now completely finished.
SVCD is also completely finished AFAIK.
No, VCD and SVCD are far from finished. Having written both, I think if you look at xine, you'll find something that a bit more complete when it comes to handling playback control menu segments and mouse hot spots. (But in many ways the xine code is not as clean as the xine interfaces in general are not as clean.)
The biggest obstacle with finishing the vlc code is being able to figure out how to write a plugin,
any plugin, that handles navigation. I am not aware of any developer documentation on how to do such . (In contrast, when I started writing a navagable VCD plugin there was some reasonable developer documentation from the guy who had just finished adding DVD navigation.)
On vlc, at present it seems like you need to write some sort of demuxer interface for MPEG streams and some sort of thread event handler and I don't know what else. If you look at the DVD code, some of it seems unique to DVD.
In the past there seemed to be not that much interest because because it seemed to felt that no one uses S/VCDs. But alas this is a chicken-and-egg problem: is there little interest in SVCD's for example because support is minimal or is support minimal because SVCD's aren't used much?