The fact that it shifts easily, quickly and adjust it’s self with no cable stretching. My first di2 I crashed, bent the rear stay, scuffed the rear derailer, and it never threw it out of adjustment. I never knew it was bent until one day I came into the local bike shop and the mechanic noticed it and bent it back for me.
Syncro shifting and ability to program it, nice but of course, not necessary. I have the wifi option set up. As mentioned, connection to garmin head units, lets you know what gears and how many times you’ve shifted. Plus wifi gives your battery level.
Also with syncro shifting, you can set it up where your right or left does all the shifting (using the etube software on laptop or phone) large and small cog, through all gears and you can set up how it goes through those gears. Even with syncro enabled, like for mine, I’ve set up the right shifter to shift through all gears but I can still use the left shifter to shift the big ring. It’s smart enough to know where it’s at and continue up and down the gears.
Shifting is a light touch and very responsive. I’ve had di2 6x and 8x versions for the last four plus years. No issues with battery, recharge once or twice per year.
New 8070 di2 hydraulic shifters are very slim even with the hydraulic reservoirs. Local bike shop mention that the shifters are slimmer for di2 versus mechanical.