The feature I praise the most? SDK.
It is excellent, elegant yet simple and gurantees that any feature I may need will be made sooner or later (or I can even made it myself).
And about buttons, seekbars... Meh... I don't need them anymore.
Although I used to be a strong advocate of adding toolbar (and still think it wouldn't hurt

) I doubt I'd use it myself. Little resource-hacker tweaks made that default "menubar buttons" fit in my foobar vision just fine, and programmable status bar with $progress() is enough replacement for seekbar (I prefer seeking with keys, just need some visualisation of seek position).
The features that could be made:
- support for some less popular (for me

) formats like wma, ra, pac, ... I'd lovely if I wouldn't have to switch to another player any time I need to play them, no matter how rarely it hapens
- display of actual bitrate not just total
- perhaps native support for DirectX plugins? Lots of them are capapble of at least 24bit processing, so quality shouldn't be an issue here..
- perhaps extend simple scripting language made for title/statusbar formatting (create commands for playback, playlist management, metadata handling and allow to attach it to various events like onFoobarStart, onFoobarClose, onTrackStart, onTrackEnd, onPlaylistInsert ...) Imagine the possibilities (!)
Edit: - and presets for DSP chains, so one can switch it faster. I hate forgetting to remove eqalizer and crosfeed after listening with headphones and switching back to speakers