The commands of the TclOO package are now part of Tcl itself. This gives Tcl a built-in object system that is fully dynamic, class-based, and includes advanced features such as meta-classes, filters, and mixins.
New version 4 of the popular package Itcl (aka incr Tcl) is also included, granting support for some traditional OO Tcl programming out of the box as well.
The evaluation of many levels of nested proc calls are no longer implemented as a stack of nested C routine calls. This revision in the internal implementation of Tcl evaluation makes deep recursion in Tcl scripts safe to do. But there's more...
Tcl delivers in the pkgs subdirectory a bundled collection of third-party packages built and installed along with Tcl.
The bundled Tcl DataBase Connectivity (tdbc) interface package makes it possible to write your SQL database-powered scripts decoupled from any particular database engine. The bundled sqlite3 and tdbc::sqlite3 packages supply a powerful and popular SQL database engine ready to use.
Both client and server sockets support IPv6 where platform support exists.
New command zlib provides utilities to handle compression of data and streams.
Temporary file creation, enhancements to list sorting and setting, dict filtering, half-close of bidirectional channels, encoding and decoding of binary sequences, finer control over load, and many many more. Every new feature is important to somebody. See Changes in Tcl/Tk 8.6 or TIP #311 for the complete list.