Not sure if it's ok to post about Melt issues here, but here we go anyway.

I've experienced considerably lower quality when playing video with Melt compared to FFplay and Mplayer.

See this comparison:

Players to the left is Melt, to the right is FFplayer and Mplayer. See the difference?
(Melt opens the clip at too large size, but that is another issue, left bottom Player is scaled down to match others).

So why does Melt have such lousy quality compared to the others?

Hi,

melt is not a simple player. It's a "multitrack command line oriented video editor".
The playback resolution is not driven by the source resolution (input track).
It's set by the final consumer of the processing queue, which gets it parameters from
the choosen profile, which is DVD PAL by default (720x576 resolution - in my install).
Calling melt with a single video as input will make a lot of use of the internal defaults
and automatic construction of required elements.

But, indeed there are some redundant scaling operations active, which can degrade image quality.
This holds for mlt 0.5. Have a look on the mlt-devel mailing list ("melt single frame pass-thru").

Toby