Without even looking at their page: logging, persistence, availability, API hooks.
Skype is awesome and is the cornerstone of my team for private communications. But if history is only transferred when parties are connected, and only between people who are explicitly added to each chat, it leaves a giant gaping hole in group communications.
Availability means it's online all the time. A server isn't, people's Skype isn't. Maybe it works for you if you're all in the same office, but I have people in 8 different time zones.
Skype is awesome and is the cornerstone of my team for private communications. But if history is only transferred when parties are connected, and only between people who are explicitly added to each chat, it leaves a giant gaping hole in group communications.