To start playback of a "container" (e.g. an album can contain several tracks, an artist can contain several albums) you need to double-tap on the container. If you tap once on the item it is selected (and the content is displayed).
To start playback of end-node items (e.g. tracks or internet radio stations) you simply need to tap once.
Note: The playback scheme was changed in software version 1.0.22 Build 3 due to customer feedback that the double-tapping for end-node items was not intuitive.