I agree that the DA-only versions of the 3rd gen S&W guns have a very nice trigger. Actually, all the 3rd gen guns have nice trigger pulls, but with the DA/SA versions (which most are) the DA trigger is not as short and light as on the DA only guns.
That is correct; the DAO S&W 3rd-gens have a preset trigger that is partially cocked by the action of the slide. The pull length and weight were advertised as roughly 2/3 of the DA pull of a conventional DA/SA Smith. The hammer is rounded and cannot be hand-cocked; it basically serves as an indicator that the trigger has been preset.
A potential downside is that the DAO guns lack second-strike capability; in the event of a failure to fire, the pistol MUST be cleared, as the trigger becomes non-functional. This is not the case with a standard DA/SA metal-frame Smith. It's mostly seen as a non-issue today, because most striker-fired polymer pistols work the same way, and clearance drills have become standard training doctrine; however, it was an issue "back in the day", when these guns were being marketed to LE agencies as an alternative to the DA revolvers their officers were accustomed to.
Another peculiarity of the DAO guns is that many of them lack the mag disconnects found on almost all DA/SA Smiths. They were built this way so that the shooter can safely pull the trigger to drop the hammer in a training or competition situation, without having to carry an empty mag specifically for this purpose.