It has big internal changes; see the change list on nitrohack.org.
As of 3 August 2013, NitroHack appears to be unmaintained and the official web site has disappeared. The bulk of NitroHack's improvements, however, can still be seen in NetHack4. The GitHub repository remains accessible.
- In-built netplay - A single account on nitrohack.org can play multiple games at once.
- Interactive replays - Replays are like enhanced ttyrecs, but information such the current inventory and discoveries can be viewed.
- Improved options - All options can be changed in-game.
- All keys can be remapped.
- Previously compile-time only options can now be set before starting a game as birth options, e.g. no Elbereth.
- Enhanced Curses UI - NitroHack uses a battle-tested version of the vanilla patch.
- Larger terminals have on-screen inventory and colored HP and Pw bars.
- Multiple glyphs on the same tile blink between each other, e.g. monster on an item on a trap.
- Visible item weights for items that can be safely moved, e.g. items that aren't gray stones or cockatrice corpses.
- BUC colors give visual distinction between blessed, uncursed and cursed items.
- Dungeon overview - The dungeon structure can be listed at a glance, showing levels, their dungeon features and annotations. Off-screen level memory can even be viewed.
- Autopickup rules - Like autopickup exceptions enhanced, replacing pickup types. Items can be grabbed/left based on name, type and BUC status.
- Automatic crash recovery - If the game crashes or the connection is lost, the game can be reloaded to the turn it happened with no intervention.
- JSON client/server architecture - This makes a web client possible (in the works), as well as game clients on any system with an Internet connection.
- Any client can watch any player, e.g. a tiles client can watch a Curses player.
- No more problems with different terminal sizes and encodings a la watching on NAO.
- Major refactoring - Support is dropped for many little-used ports and compilers, improving maintainability. This means faster bug fixes and enhancements, more outside contributions and even new features.
- CMake build system makes NitroHack much simpler to compile than vanilla NetHack.
- Variant cooperation - Daniel Thaler and those behind the current major NetHack variants (UnNetHack, AceHack and SporkHack) have said they're willing to share patches. Daniel has also said he's open to gameplay changes in the future.