Comment 2 for bug 661652

It seems as though gstreamer uses a very low quality algorithm for deinterlacing (e.g. nearest neighbor from the looks of it), which makes deinterlaced movies appear blurry and also oscillate up and down by one line.
It's certainly better than nothing, but I would opt for the use of a more sophisticated default interpolation algorithm in deinterlacing. Most systems nowadays can easily handle a more CPU-intensive algorithm, but there is no obvious way to influence the default setting.