I'll caveat this by saying this would work for me in my world. YMMV.
First, the plate carrier and soft armor carrier need to be a one piece unit. It should have the option of having plates in place or not, but the plate carrier should be integrated into the vest. If I'm reaching for that kind of carrier then I need both plates and soft armor. If all I need is soft armor concealed under clothing, then I don't need this carrier.
It should have some form of quick release system that doesn't take 9 years, a field manual, and a suture kit to reassemble.
It should be constructed in such a way as to not unduly interfere with mounting a long gun. This could be non-slip fabric, or a particular cut, but I need to feel confident and comfortable when I bring my M4 up to my shoulder.
Armor makes you larger, especially with pouches, so the whole affair should fit well to reduce the Michelin Man effect.
There must be a drag strap, sufficient hook/loop front and back to attach identifying panels, and a provision for side plates. It must also have the capability to route hydration tubes and commo wires.
As a system, the ability to attach bicep protectors, additional neck protection, etc. would be nice.
Built in M4 magazine pouches in the belly area are a nice feature that reduce overall girth compared to MOLLE pouches (at least, I think so).
I should be able to don it quickly, and it should provide adequate coverage (this is dependent on the cut and size of the soft armor).
I'm not a fan of loops on the bottom to attach it to a gun belt.
It should be made from material strong enough to be durable, but as lightweight as possible.