What would happen if it leaks DNS requests?
It will reveal the observer when your computer is requesting DNS server to resolve domain name such as google.com to IP address such as 8.8.8.8
Does that mean that snoops could potentially tell WHAT two parties are talking, but not be able to read the contents?
No, the content is still encrypted but the fact that computer X is trying to access resource secretchildporndomain.com is leaked.
What is Wireshark?
Program used to capture and analyze the data flow on network. It is called network sniffer. Requires some knowledge about networks and how to use and interpret results.
What is flawed about the SIP protocol?
It requires centralized server to communicate. Also have some problems when operating behind NAT (router). It is old-school fashion that might work exceptionally well in some scenarios but P2P technology that Skype pioneered in VoIP is the way to go.