Although FFmpeg is a very good tool, its default behaviour is not always obvious. During conversion to various video formats, FFmpeg mostly produces decent quality without any tweaking. Unfortunately this does not happen with the Windows Media Video (WMV) format. First I thought this was because WMV3 was not supported. Turns out by setting the video bitrate you can get good results (higher is better).
ffmpeg -i input.mp4 -b:v 2M -vcodec msmpeg4 -acodec wmav2 output.wmv
[Update] Alternatively you can set the
ffmpeg -i input.mp4 -qscale 2 -vcodec msmpeg4 -acodec wmav2 output.wmv
ffmpeg -i input.mp4 -q:a 2 -q:v 2 -vcodec msmpeg4 -acodec wmav2 output.wmv