Straight

This type covers the knife the most, but the knife may be harder to pull out of the sheath (you can use the lanyard), and this may not always work well for some knives with stumbstuds, opening discs or holes.

S-Shape

This type provides the perfect balance of coverage and ease of access. The knife has plenty of leather still covering it, but the s-shaped cutout allows easy access to opening hole (Spyderco), opening disc (Emerson, etc.) or the thumbstud. With this sheath you have more of the knife exposed, and you can pull the knife out already indexing its opening mechanism. And sometimes it is just more esthetically pleasing to have a gently curved top of the sheath.

Angled

This type is somewhere in between the flat and the s-shape types. Most of the knife is still covered, but the opening mechanism is usually either partially or completely covered as well.