How do you know where to put details into your illustration?

I will try to partly answer your questions (there are many!)

“It feels the design is missing some elements and I don’t know what.”

You mention you are a student. It’s common as a novice, not being able to point out what exactly is “missing” (don’t worry, it should improve with time!). Sometimes it’s not what’s “missing”. Sometimes things need to be rearranged or tweaked. I think the previous posters have offered some great pointers. However, you can have a nice illustration that looks “finished” without adding much details.

“Basically my question is: How do you when, where and what to put some
‘subtle’ details in your design to improve it ?”

Like I commented before, details are often added where you want to attract the eyes, such as in sketching portraits, eyes are detailed and the rest of the face is usually fairly rough.

The details I see added in your illustration don’t strike me as subtle. They are small details, but they do attract the eye very much because their surroundings is quite simple/flat. Thus, by addind small waves in the water, you are attracting the viewer’s eye there. Is this what you want? What do you want to communicate with your image? This is the first question you should ask yourself when reviewing your own work.

I am assuming that the lighthouse is the focal point of your image, and my suggestions below will be reflecting this. Some issues in your illustration come from contrast, some from inconsistencies, here are a few I can pinpoint:

• Make the stars a bit smaller
• Bring the moon inwards a bit (it attracts the eye very close to the
  edge of your illustration, try to avoid this if you want to keep the
  viewer in)
• Your waves are all at the same size, regardless of where
  they are; this doesn’t make sense, they should appear smaller the
  further they are. By putting some waves against the coast and some that go out of your image, your illustration would seem less enclosed in the frame.
• Shadows are not applied consistently (you have two
  light sources, the moon which should cast faint shadows and the
  shadows cast by the lighthouse beam, which should be harsher) The
  rays in the lightbeam. You might want to skip the shadows altogether, that’s up to you!
• Light beam rays should likely be in the same orientation as the beam, unless you want to suggest the beam is flashing or something of the sort.
• In terms of color values, I see the white in the lighthouse seems to vibrate with the green of the grass and it makes my eye uncomfortable. I would change one of them for brighter/darker.
• The grass is extremely bright for a night scene.

There could be more pointers but it also depends on what kind of style you are striving towards. Hopefully some of what I’m offered will help you find what you need.