1) Visual switchboard, ala mysipswitch - so calls can be fiddled around with 'in transit'
2) star (*) codes for activating services - i.e. we can use the web GUI to setup cfwd rules, but have to be on the web to activate/change them - hitting a * code and a given number would be handy
3) Ringback - although admittedly you'd have to have a bunch of regional PSTNs/freephones for that - but it's a very handy thing to have (but a pain to configure via other sites currently!)
4) Mobile web interface supporting HTTPS (rather worrying it doesn't so far!)
5) API access to premium users (I have fudged this via the mobile web interface but it's not secure and less than ideal!)
That's my 2p worth for now!