I just went through all wishlist threads and tested the Android app. The killer feature that would convince me of switching from Spotify + dbPoweramp to the foobar app is an integration that allows switching the playback from the desktop to the mobile app - and the other way around.

The way Spotify implemented this function is just perfection, I use it all the time - within my four walls (e.g. living room -> bathroom) and especially when leaving home (desktop) and just continuing to listen on my smartphone.

I was surprised that nobody suggested this yet, since I think it is very popular due to Spotify and I've been wishing this since day 1. This thread comes closest. The user suggests a synchronization between the Desktop's and Mobile's database.

I see two implementation scenarios and for the first one, database synchronization is a preliminary step:
1) Folder(s) exists on all devices and the databases are synchronized. A UID identifies each file uniquely and the current playlist is synchronized, allowing seamless playback. Optional: If a file does not exist on one device yet, it is replicated/streamed just in time and added to the devices database.
2) Playback is continued using a streaming technology like upnp or an http-server. The desktop app probably already comes with all necessary prerequisites.

An optional feature, or maybe also light solution, would be to use either the desktop or mobile app to remote control the playback on the other device. However, for controlling the desktop app (the more common case, I think) there already is the really good mobile app.