I was gonna add one more thing but managed to forget: you may be aware that we also sell DC-powered OpenSprinkler (microcontroller-based one, not OSPi unfortunately). This is my own creation, while all other sprinkler controllers on the market use 24VAC and triacs. The thinking behind developing DC-powered controller is that I consider 24VAC an old technique that should be obsolete. 24VAC transformers are bulky, expensive, and not universal (i.e. same transformer does not work in all countries). On the other hand, DC power adapters are much smaller and cheaper. The DC-powered OpenSprinkler uses MOSFETs, which are much more efficient switches than triacs because, as you mentioned, the voltage drop across a MOSFET switch is very very small so they can run a lot more current.
I have always hoped that the industry will move more towards DC-based system, but it seems no one has any incentive to replace them. Some companies like US Solids, have created motorized ball valves that work with a wide range of voltage (like their 9-24V AC/DC series). I am really hoping to see more of these changes and innovations, because I believe 24VAC should be made obsolete and replaced by more modern systems.