Last time I checked an M800 was nearly 3 times the price of an S80 - and to get the same features you need to pay more for option codes to unlock the features you require - this is not a bad thing in itself, but you are comparing the DTA with a different end of the market. M84 is still more expensive, the main advantage I see is onboard Lambda control (wire ecu straight to Lambda sensor).
Realtime/live laptop keyboard tuning would be a feature that I would like to see implemented (even in the next series of ECU) but having said that now that I have tuned one using a knob box I certainly prefer that over trying to hit keyboard buttons.
One of the reasons is possibly the perceived advantage rather than actual usability increase. Like wanting Sequential instead of batch/group injection when the injectors are at 90% duty anyway haha - no real advantage yet it sells units!
For the price point I think they offer great features, especially since the last update which divorced the Aux outputs from Analogue inputs if required.
The only things that spring immediately to mind would be:
- Aircon request/output control based on selectable load/rpm
- Stepper control would be nice.