It can connect to multiple SIP-PSTN gateways, all at the same time, and receive calls any of them at any time. (This also eliminates most, if not all, of the existing ones.)
When a call is not in session, occupies zero or nearly zero screen real-estate, and very little memory, just the basic SIP listener.
Uses the local system appropriate address books, such as the Mac address book, the Outlook address book, and the ones from Thunderbird, Evolution, a directory full of vCard files, etc. Without "importing", it just uses them where they are.
If the system address book apps allow plugins, it plugs into them without fuss. Maybe just put a tiny "call this number" next to each displayed telephone number or SIP address.
Plugs into Firefox, Safari, Mail.app, IE, etc, so that anything that looks like a phone number or SIP address has a tiny "call this number" icon
It does SIP without fuss, from the most simple setup to complex SIP configurations.
It can do direct node-to-node SIP calls without special setup.
Tries to negotiate SIP and RTP encryption whenever possible.
Can keep a full call history, and lets me attach notes and comments to each call, just by clicking on the the call log record.
It can record calls, either at setup, or at a click while the call is ongoing.
Records the call setup data, caller ID data, and other SIP metainformation in the recorded call file, as well as in the call log.
All call logs and configuration files are in easy to read text form, such as just text or XML. Recorded calls are WAV files kept either in standard uLaw encoding, and/or the codec of the call.
Can handle all the well known codecs without fuss or special additional software installations.
Is intelligently aware of the "away" status. If the screensaver and the IM client can figure this out, the softphone can too.
Can act as an answering machine, when I am away, otherwise don't answer, or just push a "send this call to voicemail" button.
Missed calls because I am away generate an email.
Can automatically mute or pause the music player when a call is in session, for the various well known music players.
Can keep a PSTN/SIP bypass table, so, ferex, when I call a Earthlink Speakeasy phone number, instead of using a PSTN gateway, just directly connects to Speakeasy's SIP server. There exists such a table online, it just needs to check and download a new one every 30 days or so.
Can announce itself with Bonjour.
Can discover local network PBXs via DHCP and via Bonjour.
Has a "setup wizard" for FWD, Gizmo, etc, and all the well known SIP providers.
All this doesn't seem that hard, and yet, I can't find something anything like this.