Complaining to ASUS hasn't done an awful lot so far - your best bet is to install a custom firmware on it - that'll get most of what you want working.
You can get FTP going without changing the firmware - you need to open up ports 20 and 21 in your BT hub and forward them to the IP address that the hub gives to your wl-700. This means that when requests come in on those ports (the FTP ports) then the hub will pass them onto your wl-700. I recommend you use UBSA mode to password protect your FTP server though.
There is a better itunes server called mt-daapd if you install a custom firmware - this will pass on ID3 tags to machines trying to play music off it. There is also effort into writing a php frontend to a torrent client for the router, so you can download torrents via a web page - have a browse of these forums to find more information on these things.