I've got some questions:

Is it possible in DirectX 11 to set up a program without setting the hole graphic pipeline (e.g. Vertex-, Hull-, Domain-, Pixelshader)?
I didn't find a way to get a triangle on screen without it. (might be an objective point in the "first contact is easier" discussion)

Furthermore: many posts here say that GLSL has a lot of "potential silent failures". How do the existing programs (prof. OpenGL programs as well as OpenGL-games) handle this? I never had problems in running such programs on my computer.
Or is this fact just due to some drivers accepting more than the standard and others seem to have bugs if they stick to the standard. (whatever the standard is...)

Thanks ;)

You need vertex and pixel at a minimum, but then again the same also applies to a modern core OpenGL context (where VBOs are not optional either).

Real commercial programs avoid problems (or at least try to avoid them!) before they happen through testing, bugfixing and working around identified driver issues. If a given combination of blend modes and texture setup doesn't work on an ATVIDIATEL driver, even if the spec says that it should, then you insert a lot of cruddy code to work around it. Because people won't blame their driver or it's manufacturer for not working ("everything else works") - they'll blame your program.

See for example the Autodesk Inventor PDF link I posted earlier - at the time of writing, despite only using 1997-level OpenGL features, and despite having had 10 years to solidify their code base, they had 44 driver workarounds.