People are often comparing GnomeMeeting to the Instant Messengers concept which is, I believe, a wrong approach. I guess it is the result of the marketing efforts of Microsoft and Apple for that kind of solution, but I think that it is time to react to that situation and give a clear view of what GnomeMeeting is and what GnomeMeeting is not.
Most of the commercial implementations of Instant Messengers offer the possibility to send and receive video and voice using a myriad of different protocols and codecs, most of them being proprietary. In my opinion, although it is something good for the average user, I also think that people need more serious and complete solutions if they want to replace the phone they are using every day by something more powerful and more ... modern.
GnomeMeeting is thus a modern IP softphone, with the aim of replacing your classic POTS (plain old telephone set) or your cellular, and use the Internet instead of classical phone lines, even though it can of course be used to call normal phones too. An Instant Messenger will only have very few features compared to a cellular and will very likely be unable to replace it one day.
GnomeMeeting is based on H.323, an IP telephony, Voice Over IP, and videoconferencing protocol. It permits interoperability with H.323 compatible software and hardware (see for example: http://www.swissvoice.net/ww/htm_ww/07_products/ds_ip10.html, that is an H.323 IP phone). You can use GnomeMeeting to call other IP softphones, or hardware based IP phones, but also to call normal phones on condition that you have the required hardware and/or software infrastructure. A few companies are starting to use such solutions. Imagine a company having appropriate hardware and software connecting the classical phone or ISDN lines and the Internet. When a customer calls the company for support, he is confronted with a vocal menu asking him to do a choice to either contact the commercial or the technical support service. Once the choice is done, a technician answers and helps him. The technician can put the call on hold (you hear the music), or transfer it to another technician with more competence in the field of the customer's question. Those technicians will be using IP phones or softphones (like GnomeMeeting for example) connected to the LAN of the company and it will be transparent and efficient for the customer. You could not do the same with an Instant Messenger like iChat or Windows Messenger, both types of software are not comparable and should not be compared because they do not have the same purpose.
To summarize, GnomeMeeting has many required VoIP features permitting it to transparently replace classical phones and cellulars in VoIP infrastructures : compliance with standards, Call Transfer, Call Forwarding, Call Hold, possibility to register to gatekeepers managing calls, possibility to be controlled by classical phones plugged into Quicknet hardware, dialpad simulating the dialpad of phones (if you are presented to a vocal menu and have to make a choice)...
GnomeMeeting will never be a simple Instant Messenging solution with a few videoconferencing features, because I still believe in the day when people and companies will use IP phones and softphones like GnomeMeeting instead of their normal phones to save the costs. Of course, all of this does not prevent Instant Messengers from interoperating with GnomeMeeting to offer the same kind of "simple" features as in the Windows and Apple worlds, but we do not have the same goals.