The big difference between operating in straight D as opposed to GAAP D is caused by the VMC criteria.
In GAAP - clear of cloud.
In Straight D - 1,500m horizontally and 1,000 feet vertically from cloud.
This means with a base below 2,000 feet in straight D over a built up area all VFR flights are effectively required to obtain a Special VFR clearance. Whilst positive separation is not required between two special VFR aircraft (only due cloud and only in D), it is required between a special VFR aircraft and an IFR aircraft. Traffic advice (sight and avoid) is not an option where an IFR aircraft and a special VFR aircraft are involved, and given the physical size of the zones I think you'll be able to work out where this is will lead when the weather is a bit average.