Pretty much sums it up.
A big influence on drainage is the pop, rectangular pops are much narrower and have a greater percentage decrease in width at their narrowest point than round pops have. This not only effects the initial flow (fall) characteristic, it also means that rectangular pops are less likely to discharge debris. Gum leaves in particular can mesh and cause blockages. If gutters were better designed, the section where the downpipe fitted would flare and this would then allow larger conical pops to be fitted.
Regulation is also needed to address the problem of roof litter ending up in the waterways. Melbourne Water is promoting a Raingardens program to raise awareness of this issue.
http://raingardens.melbournewater.com.au/