What determines whether a style of meditation can or cannot lead to insight?

Supposing you thought of a coloured-disc - then, the disc is not real - simply because you don't experience a disc - what you experience maybe one or more of the following(one after the other): perception of colour, perception of shape, perception of any of the four elements, mind-consciousness, pleasant/unpleasant/neutral feeling etc - and the above are all namas - that's what you experience, and that's what is real - You do not experience a "disc" - hence it is not real.
Now, when it comes to Samatha Meditation - As far as I can see you choose a particular concept - and stabilize it in your mind - to access jhanas and so on. As far as I can tell - the difference isn't really in the object but how you use it - for example - you take up a coloured-disc in your mind and then analyse the three characteristics of the namas (their impermanence, non-self and suffering) - and that would be vipassana instead of the usual samatha it is used for - but it seems, as Yuttadhammo mentioned somewhere on this site - that some objects of meditation are far better suited for samatha and some others for vipassana.

Tranquility meditation is a method, and it has nothing to do with an object. Tranquility meditation can easily use the rising and falling of the abdomen as an object, just like insight meditation can easily use a concept as its object.

Now I can see conceptualization itself being an issue, but again, that's nothing to do with tranquility or insight.

If I am to interpret meditation leading to insight as meditation which takes you to the final goal then it should have the following characteristics.

• should be based on the foundations of mindfullness
• you should come to a station where you experience the whole body,
• you should calm the body conditioner
• you should look at sensations cultivating the factor of rapture and joy thus calming the mind conditioner
• etc.

Meditation devoid of the above may not take you to insight. Even if you take to develop concentration still you have to pass through these stations.

Tranquility meditation requires focus on a concept

Tranquility meditation such as Metta or Recollections of the Qualities of the Buddha are based or expanded on concepts.Tranquility meditation using the breath is based on the "real" " physical" " direct" sensation of " **touch ** " in the nostrils.

Is the reality of the meditation object the primary difference between meditation intended for tranquility and intended for insight

Of course you'd want to choose a meditation object that you can concentrate on. The more real the better. The more solid (rupa) the better.You need concentration to develop insight.So the 'realness' of the meditation object is not the "difference" between tranquility and insight but is in the best interest of both meditation practices.So my answer is no.Meditation objects are not the primary difference between tranquility and insight its the meditation technique one uses on them that differentiates the two.

How is mentality real while a concept is not?

Forgive me.I don't know how to explain this but ill give an example: Loving kindness is a concept.Sukha and piti are nama.Nama are like the actual real mental qualities that arises in our mind and concepts is almost like a label we give to the combined mental factors.Since insight deals with seeing things clearly it sees straight through concepts and deals directly with Nama and rupa.Tranquility however doesn't have to break the concepts into nama.it just has to stay with it for however long it lasts.

Here is the difference: a thought itself is real, but the object of the thought is conceptual. Loving-kindness is real, but the object of loving-kindness is conceptual.

When thinking, one is really thinking. When thinking of a cat, the cat is not really there, but the thought is.

When loving a person, the love is really there, but the person is only a concept.

Mind refers to the process of thinking, loving, etc., itself, not the object of the mental activity. When the mind focusses on the movements of the body, the movement is real, the focussing is real, but the body part itself (e.g. hand, foot, stomach, breath etc.) is a concept.

If one focusses on the breath, for example, going in and out of the body, one is focussing on a concept, as it is not directly related to the observation of the sensation. If one focusses on the sensation at the nose or the abdomen, then one is focussing on ultimate reality.