OK... firstly, Skype is not an instant messenger with voice chat capabilities, technically it's a voice chat program with instant messaging since it was designed from the ground up for VoIP (a mere technicality I know, but I'm like that).
Next, I'd mention something about OS and server support. As far as I know, RW is the worst with only a Windows server and Win/Mac clients, Ventrilo has multiple server support but only Windows client, and TS has both Windows and Linux server and client. Skype is peer-to-peer so no servers, but it has clients for pretty much any platform imaginable.
Next, perhaps a bit more quantative analysis of voice quality. This needs some audio analysis software and a fair bit of time, and is probably too much effort for too little gain. Still, I wouldn't mention at all what they claim to be their sound quality, rely only on what you hear. CPU usage might be a concern too, especially for gamers who prefer their CPU to be working on the game more than anything else.
Finally, I'd have a short list of features for each one - your glossary would be a good place to start. For example, Skype doesn't have push-to-talk or voice activated, it is always on, which can be a pain sometimes.