One thing that will be in KDE Plasma Desktop 4.9 is a new version of the Now Playing widget. Based on QML, it works much better, especially on panels (where its design is based heavily on my favourite KDE-3-era applet, kirocker).
In the background, it uses the new mpris2 dataengine. This exclusively supports MPRIS2-capable media players (which these days is most), and doesn’t include any hacks to support XMMS 1, for example. JuK and Dragon have both gained MPRIS2 support for the 4.9 release, and Ubuntu’s sound menu uses MPRIS2, which is compelling many other media players to support it.
The result is a cleaner, more reliable design, and completely asynchronous behaviour, so it will use less power and (unlike the old nowplaying dataengine) it should never cause your desktop to freeze due to a badly-behaving media player.
The old nowplaying dataengine is still there (and will be until KDE Plasma Desktop 5 is released), but should not be used for new widgets. Instead, you should use the mpris2 dataengine, and I highly recommend porting any existing widgets to mpris2 as well.
Overall, I’m really pleased with the design of MPRIS2, which allowed me to create the mpris2 dataengine with a minimum of fuss, and allowed for widgets to implement a seek bar without querying the media player once or twice a second to find the current position (and not take the performance/power hit if they didn’t care about the current playback position).