- The collision detection has been improved and can now handle speeds beyond the previous limitations. Super Sneakers and spin dashing have been dramatically sped up as a result.
- Many bugs involving the controls beyond normal maximum speed have been fixed. This should make the controls feel more responsive when moving extremely fast.
- When holding back, the character now skids to visually and audibly demonstrate that simply letting go of forward doesn't slow the player down as much as reversing direction.
- Dropped rings now travel farther and become harder to recover when taking repetitive hits.
- Knuckles has been updated with lavish new sprites.
- Monitors have all-new sprites, and shields now all have individual sprites instead of being palette swaps of each other.
- You can now record Animated GIFs in-game.
- Techno Hill Zone Act 1 and Castle Eggman Zone Act 2 have been dramatically updated.
- Techno Hill Zone Act 2 has been replaced with an all-new act.
- Castle Eggman Zone Act 3 has been added with an all-new boss.
- The special stages have been completely replaced with new stages based on NiGHTS into Dreams.
- All the rest of the stages have been given a fresh coat of polish.
- The final boss has been replaced with a new, actually fun design.
- The emblem system has been completely revamped, with hidden emblems accessible as all characters. Explore for emblems however you'd like.
- Record Attack, an expanded version of Time Attack, lets players set records in time, score, and rings for each act they've accessed in the main game and record a replay of the run. Beating the requirement in a category for a stage grants an emblem.
- Record Attack replays can now be raced as ghosts in Record Attack, allowing players to directly see the difference while playing. They can also be saved as a guest replay to send to a friend's copy of the game for them to race against.
- NiGHTS Mode allows players to set records on special stages similarly to Record Attack. Sadly, NiGHTS mode does not support replays and ghosts in this release.
- Sonic can now have Tails follow him around through the single player game like he can in Sonic 2 and Sonic 3.
- A large amount of multiplayer stages have been added.
- In match and capture the flag modes, players now spawn with basic shields and losing players get free basic shields during the round.
- The grenade ring has been completely redesigned, and the infinity ring from older versions has been reintroduced.
- The old Classic Race gametype has been revamped and renamed to Competition. Similar to Sonic 2's versus mode, players race through single player stages and attempt to grab goodies in the stage before completing the stage. This gametype now actually functions well beyond two players.
- The software renderer now features skyboxes, allowing for complex, animated skies by rendering another section of the level as the sky.
- The system for adding custom textures has been changed to a very rudimentary implementation of ZDoom's TX_START/TX_END and TEXTURES lumps, so that two WADs with custom textures can be added to the game at the same time without issues.
- A rudimentary version of ANIMDEFS has been implemented for animating textures. Support for Boom-format ANIMATED lumps remains.
- Tall patches are now supported, allowing images larger than 255 pixels tall to be imported to the game as a single image.
- TypeOfLevel can now use gametype names instead of requiring numbers.
- Level headers for levels 100 and above can use letters instead of the decimal equivalent; eg Level A0, A1, AA, ZZ etc.
- SOC format (a DeHackEd variant) now supports internal thing names, other enumerations, and basic math. Porting mods to future versions of SRB2 will be easier, as these names rarely change.
- blarrg's Game Music Engine has been implemented, meaning you can now use rips of NES, SNES, Genesis, etc. soundtracks as music in-game.
- There are now 25 different skin color options for characters to use, including returning colors from older versions such as Dark Red and Black.
- Super forms for characters now glow like they did in Sonic 2 and Sonic 3&K, allowing yellow to be usable once again for match mode. Super Knuckles as a bonus flashes pink instead of yellow, but this is not available for custom characters.
- Multiple characters can now have the same displayed name; no more "Tails2" if you add a custom Tails character.
- All SOC actions and extra character abilities from SRB2Morphed have been imported, allowing SOC to be even more versatile.
- Colors, abilities, and spin items are set by name, as with SOCs, to enable forward compatibility. If the color numbers ever change again, the intended color will still be used.
- A scripting language based on Lua has been added to the engine exclusively for use by mods. It can be used to create entirely new console commands and thing behaviors in game, allowing a lot more flexibility for mod content without requiring editing SRB2's source code itself.
For now, 2.1 is only available via torrent. It will be made available on http://www.srb2.org/ at a later time. Source code is available here.