1. The ability to monitor in both SDR and HDR without baking in the look.
“The first ALEXA, now known as ALEXA Classic, introduced in 2010, and all successive ALEXAs (as well as AMIRA) provided a LUT-based output transform for REC709, which allowed for SDR (Standard Dynamic Range) monitoring of a default rendering of the camera’s footage on standard production monitors and televisions. As HDR monitoring appeared, ARRI created new LUT-based output transforms for that viewing environment (such as HLG and REC.2100, also known as SMPTE 2084 or simply ‘PQ’).
However, only one LUT at a time could be applied and viewed, and any creative choices were “baked,” or married along with the output transform. With ALEXA 35, both Look files and output transforms are handled in a new way. Previous ARRI Look Files (such as ALF-3, first used on the AMIRA and ALEXA MINI) took a Log image and corrected it for either SDR or an HDR viewing environment such as PQ or HLG along with any creative look choices. In other words, both the creative color information and the output transform resided in the same file.
The new look file format introduced on ALEXA 35, ALF-4, goes from LogC4 to LogC4, a so-called “Log-to-Log” transform, which provides a layer of abstraction for creative color choices without being tied to an SDR or HDR rendering directly. An output transform then follows this “slot” in the camera’s processing for the viewing environment being used. Different output transforms can now be applied to SDI 1 and SDI 2 to facilitate side-by-side monitoring of SDR and HDR on set. The important point is that the DP or colorist’s creative intent stays in the look file, allowing the color information, and creative intent, to travel independent of any color space, or output, transform. This paves the way for any future color space transforms to be able to be applied to the signal thus preserving the creative look.” – AbelCine
The ability to monitor in both SDR and HDR is critical for high-end productions. But even for budget work it’s awesome, and one of the things we like most about the Komodo, which allows monitoring in SDR using a smartphone and the RED Control app while sending out an HDR signal over SDI to an external monitor without the look being baked into the footage.
2. The handling of bright saturated colors.
Prior to the new REVEAL color science, saturated reds would go bluish (see photo above). Now, highly saturated colors like neon signs or car brake lights are captured with greater fidelity, as are typically challenging colors like cyan, burgundy, and pastel shades.
3. No longer desaturating colors above middle gray.
Back in 2014, Art Adams authored a much quoted article in which he vaunted the way ARRI cameras desaturated hues to mimic print stocks.
“Alexa color saturation locks once the luma value of any hue as it approaches middle gray. (Middle, or 18% gray, falls at 40% on a luma waveform.) As exposure increases beyond middle gray a hue will brighten but its saturation will remain the same. By decoupling color saturation from luma beyond a certain level Arri seems to be deliberately playing against video’s strengths as an additive color system, and I think that’s a key aspect of what sets them apart. Video has its own look, and it’s a pleasing look in its own right, but bright saturated colors tend not to appeal to visually sophisticated viewers. Film’s approach to color has more in common with painting than TV displays, and it’s an aesthetic that greatly appeals to many in our industry… including myself.” – Art Adams, DVInfo.net
The new color science, looking toward the future with HDR, allows for bright, saturated highlights, which can always be toned down in post-production if necessary.
4. The possibility of ARRI adding a look with less saturation to the Alexa 35 in the future.
Art Adams signaled that ARRI might add a look with less saturation to the Alexa 35 if there is enough interest.
5. Larger in-camera LUTs
The Alexa 35 accepts 65 X 65 X 65 3D LUTs, which is awesome, because for 12-bit HDR, larger than 55 × 55 × 55, if using the tetrahedral interpolation, should result in unnoticeable interpolation errors. To the best of our knowledge, most cameras only work with 33 X 33 X 33 LUTs.
The Daejeon Chronicles 
going from mirrorless to let’s say RED is a night and day regarding video fidelity (highlights, color control) but going to ARRI from RED is like… I blanked out trying to come up with something witty but regardless. The only thing limiting the sensor is the price but if this sensor was in every smartphone magically overnight you couldn’t tell the difference because the content is consumed differently.