My personal favorites are a rotary twist compared to a lever (especially those rotary ones that you can extend the knob size, like the Golem or the PSCR-types). I have yet to find a lever that one can easily switch single-handed. A rotary design needs to be REALLY stiff to require two hands. Some lever-actuated ones are ok if they're part of a BOV where you have a large moment arm to brace against in order to flip the switch from loop to OC, simple DSV's like the ISC or SF2 ones are virtually impossible. It also serves as an easy visible indicator that the DSV is open or closed. With a rotary, if it's parallel, it's open, if it's perpendicular to gas flow, it's closed. With a lever, depending on the position, it's not as visible. I even saw one diver attempt to install his loop backwards because he thought the DSV was open based on the lever position, he simply had it upside down and the way it was hanging fooled him for a second. Not a real world issue so much, but it's something to be aware of.
I'm not sure some brightly colored indicator would be worthwhile. I'm not sure it would really hurt anything, but people disregard their HUD's, I don't doubt that they're quickly displace a brightly colored DSV switch from their consciousness.
I like bayonet connectors. They solve the issue of clocking the mouthpiece, ease of assembly and disassembly, and facilitate better loop cleaning since you have full access to the hose, the DSV, etc. A good many are simply hose-clamped directly to the DSV without any provision for ease of disassembly.
Ease of service will depend on the DSV design. Some are easier than others.
I'm not sure what you mean re:modular mouthpiece. All of the ones I've used take a normal mouthpiece and use a standard zip-tie. I personally spread a thin layer of silicone underneath the mouthpiece just to make sure there's nothing that could affect its ability to seal. It's frustrating finding a leak if it's coming from the mouthpiece stem. If you mean modular like the Golem where you can unscrew 4 screws and screw in a p-port adapter for FFM use, I'm not familiar enough with the other manufacturers units to tell you that this is common or not.
As for hose size, a modular one like the Golem or the Divesoft allows you to pick the matching hose nipple for your loop hoses. It's a pain trying to stretch a set of Coopers onto a connection that's too big, I can tell you that first hand. And because they are modular, you can choose loop hoses independent from what the unit is designed to use. I personally like Coopers, so I get the Golem hose nipples for Coopers and don't have to worry about it.
My personal choice is a Golem BOV (I'm a BOV advocate) but if I had to choose a DSV only, it would also be the Golem. I have minimal experience with the Divesoft, and I've heard mixed reviews, although I'm not positive that those were not early models and have had issues ironed out. I made my choice purely based on my own opinions about what is important to me. Other divers may choose something else for their own perfectly valid reasons. The ones I absolutely don't like are the whole barrel twist ones.
