A few more features fresh in my memory from the wikibinge:
- Many new compatibility options (the aforementioned decoration shoot through is just one of them)
- Many, many new DMFlags for cooperative plays, such as having to kill all monsters before leaving the map or seeing the other players on the automap.
- A teleport special that preserves momentum
- Functions to handle weapon reloading through DECORATE
- Customizable Hexen bridges
- PlayerInput ACS functions
- Get/SetActorProperty can tell/modify whether an actor should behave like one that was in the map since the beginning (including respawns for weaponstay and such) or not
- New aliases for IWADs
- Support for extremely fast custom projectiles
- Many actions are now more flexible through optional parameters, for example A_Die can specify damage type to send to a custom Death state, A_KeenDie can affect tags other than 666, A_Fire can specify a vertical offset, and so on.
- Enhanced Strife support
That's about all I remember that isn't already listed by Graf.