Gapless Playback

Just about. There's generally a couple of milliseconds between tracks, if I remember correctly. We might have the CPU headroom to crossfade -- I don't remember. This won't happen for a while, however.

Filter mode / Refining searches

No. We don't have this. Figuring out the UI shouldn't pose too much difficulty, but the server's database doesn't currently support more than two 'and' clauses.

Streaming Audio

On the list. Technically possible.

Browse while Play

We've always had the ability to use the UI while playing. Don't know if insert-after is in the receiver software. It's in the current development build, so it'll probably be in the next release, whenever that is.

Web Server

I've been toying with implementing a web interface to the server on the PC, which is not the same thing, but some people might find it useful. Don't know if it's worth the schedule time, though.