(not exactly a visual design issue, but certainly a usability issue)

The Mediacontroller Plasmoid is used for quick access to a media player, to quickly skip to the next track if you don't like the one playing, or the previous one if you liked it so much you want to hear it again, or to quickly pause if e.g. the phone rings and then unpause again.

Stopping as opposed to pausing was important for cassette or optical disc players because they continued to consume energy in pause mode, but for a digital media player, this is not the case. Therefore, the usecases for stop are pretty rare in a digital player (for example if you take a longer break form listening and afterwards would like to resume the track from the beginning).
These usecases can well be served from within the player's main UI.
Even many players (e.g. Amarok) do not have a stop button in their main UI by default anymore, so if even they don't have it, why would a quick-access Plasmoid with only the most important features have it?

Stop may seem like an important function if you're still used to using it, but if they don't have it easily accessible, most users realize sooner rather than later that they don't actually need it all that often. So, the media controller Plasmoid is not the right place for it.