End-to-end connectivity is a property of the Internet that allows all nodes of the network to send packets to all other nodes of the network, without requiring intermediate network elements to further interpret them.
Recently many network elements and technologies that limit this property of IP have become fashionable such as network address translation, requiring network elements to support each individual protocol that is used on top of it. This hinders deployment of new applications that use the Internet in ways other than opening TCP connections to other Internet hosts. Examples of hurt applications and protocols include IPSec, migration to IPv6 (tunneling IPv6 in IPv4), Peer-to-peer applications, and networked games.
Increasingly this trend divides Internet users into those who have "real" Internet connectivity and those who are restricted to use applications that only use outbound TCP connections.