Coastlines

Coastlines are a direct reflection of Geography and should be treated as the most important aspect of geography as a whole.
Coastlines can be rough, curvy, straight, or even boring, but in order to achieve realism, the key is on trying to find repeating patters in our world that can be applied into our created worlds.
A very common mistake is to make all coastlines within a world look similar, which ends up creating a false sense of cohesion while taking away the realism aspect of it.
Another important key is to have repeating elements while having them appear seemingly randomly to the viewer. River deltas, for example, are very uncommon in temperate climates, while long rocky coastlines with small beaches are really common, in said climate type.
Yet another common mistake is that of taking very uncommon features and placing them everywhere, with river deltas and island arcs being two noticeable examples. Even when placed in zones where they are likely to appear, one can only expect such rare and unique features to appear every once in a while, with most rivers not having deltas and most islands not being on volvanic hotspots leading to the stereotypically Hawaiian island arc.
One also has to take into account local terrain: The Netherlands has a very low and plain terrain, reason why the coastline is bland and has an inmense lack of features, making it essentially be long barely curved lines. Western Norway, on the other hand, has very rocky terrain which has seen the fury of the Northern and Artic Oceans, as well as a cold and rough climate, reasons why the coast presents thousands of fjords, some of several kilometers wide and others only a couple meters wide.
The coastline directly reflects the terrain in the area, being itself an arbitrary line drawn by nature where an otherwise sloped terrain turns from underwater terrain to dry land, reason why the underwater and dry terrains near the line of coast must always be considered while drawing it.