There's no demo for Decisive Action, unfortunately. Bug Jim Lunsford about that.
Highlights of the changes to the CGSC version include:
ATO tool: You must plan all your air missions on the setup and 0000-0200 turns. Missions are either Interdiction (with a target point and a planned target type) or CAS with a specific supported unit. On other turns, you can change interdiction to CAS, and who CAS supports, but not the timing. If there's no appropriate target at the time slot, then you lose the sorties.
Logistics: Now it's either fully manual, or not modelled at all. Convoys are no longer automatically generated, but are special units in the game with a carrying capacity. Supply consumption rates can be set by unit, though we're really only worried about Class III and V (fuel and ammo). Convoys can be airlifted, and can have escorts assigned of up to battalion size (any ground combat or MP unit). We expect the loggies to be fully employed in our exercises; a division needs a minimum of about 20-25 convoys minimum running constantly.
Artillery is rather more complex, with more linkages for GS fires, more restrictive modeling of SEAD, and the option to require a counterfire radar to activate CFZs and CFFZs.
TAIs take 6 turns to become active if not placed at setup. [Though there's wrangling over this.]
UAVs only search at a single designated point in their flight path.
Trouble is, the AI hasn't got a clue how to use most of this (it cheats to get around this in our version, since we hardly ever use the AI except in testing); the coding for it would be quite complex, and Jim's kind of crunched for time.
None of the changes address some of the concerns you guys have about making shell units or breaking things down to companies at will etc.