That sort of moth-eaten coast is what I would expect of a sediment-starved coastal marsh that's actively subsiding (e.g. the coast of Louisiana). The large areas of uniformly tiny islands at a uniform distance from the coast are more disturbing to me (I am familiar with barrier islands, but they tend to have a different proportion and distance distribution). It's a good first-glance plausible result, but I'm not sure how much I can really say about its "naturalness" without a scale and other details such as water flows. Overall a good multi-fractal synthesis technique.

The big thing about this coastline is that it's easy enough to rationalize the "naturalness" of parts of it in many ways. However, each section of the coastline seems to be at a different scale and type than others. Hard-rock coasts like Canada or Norway have a different character than soft sediment coasts like the US Gulf Coast or Bangladesh. A view from 5 miles up is quite different in character than one from 100 miles up, no matter where you are. I often see maps here where one set of features don't match another (for example, a fractal forgery that works for a 500 mile stretch of coast that's then modified by a fluvial model that's appropriate for a 1 mile stretch of coast). Most people won't notice it, but I've made too many bad coastlines over the years and have an unfortunately large library of incongruities available for comparison. My sins on the FT terrain synthesis and river finding still haunt me...