I've been thinking about this thread quite a lot since Geoff invited me to comment. I've not read the entire thread. Forgive me if I cover material that's already been brought up.
I suppose the reason this has taken a lot of thought is that although I've been practicing for approaching 10 years, Jhana has never been something I consciousnly cultivated nor thought about to a great extent. Nor have I had a very clear idea of the definition of the various Jhanas and how, if at all, they related to what I experienced on the cushion. There is no doubt that it is significant in the Suttas, and of course I'm aware of samatha as an aspect of meditation, however my background in Zen (even though the root of the word 'Zen' is 'Jhana') and in secular mindfulness meditation (using techniques from the Vipassana/Insight Meditation tradition) it has never been a specific goal. Also, I consider myself a relative beginner when it comes to the Nikayas. So, this is not 'Jhāna According to the Pāḷi Nikāyas' so much as 'Jhāna According to my experience of meditation in various traditions'.
I have experienced all three of the states described here:
(i) attention training where one absorbs into a single object and thereby stills all mental factors to the point where, as Ajahn Brahmavamso explains, “Consciousness is so focused on the one thing that the faculty of comprehension is suspended … there is no comprehension of what is going on.”
(ii) attention training where one attends to a single object and thereby calms and unifies all mental factors to the point where, as Leigh Brasington explains, “It is possible to examine the experience because the state is so stable and self sustaining on its own.”
(iii) attention training where one attends to whatever occurs in the present moment (either with the aid of a support object such as abdominal movement, or choiceless awareness without the aid of a support object).
Putting aside visualisation for now, I see meditation as divided into two main types:
'Concentrated': where attention is directed towards a specific phenomenon at a time, usually in a sustained way, for example Mindfulness of Breathing
'Open': where attention is on present moment experience, but not limited to any one phenomenon or type of phenomenon, for example, Shikantaza, Choiceless Awareness
There is also a varying extent to which experience is absorbed or to which we open it to 'meta-cognition' - a sort of 'spaciousness' or 'detached awareness' which can allow us to recognise or understand truths about our experiences.
(i) above is absorbed concentration.
(ii) is concentrated meditation with meta-cognition
(iii) is open awareness
As I see it
(i) is the type of meditation most conducive to Samatha and to Jhana. I think 'Samadhi' also refers to such states. I see Jhana as an altered state of conciousness, a type of trance state even, in which the quality of absorbed concentration is very high. I have not perceived a great deal of value in such states as they are necessarily impermanent. Although they may be highly blissful or tranquil, unless there is some kind of 'learning' or lasting insight to prompt a permanent shift in one's relation to experience then any 'benefit' is necessarily short-lived. It may help with concentration skills. And insights may naturally arise from it. It has never been my goal to attain rebirth among "the gods of Brahma's retinue".
(ii) is most conducive to insight/Vipassana, as there is a 'space' for making observations about one's own experience
(iii) is Choiceless Awareness or Silent Illumination/Shikantaza type zazen. I see this as having aspects of both Samatha and Vipassana ie. absorbtion and meta-cognition.
Reading descriptions of Jhana in the suttas I recognise them in my own practice to some extent at least. However, I recognise that in Theravada it's considered bad manners or bad practice to talk about one's own experiences in such a way lest it be taken as bragging about attainments. I don't see what is being described as four distinct states, but as increasingly absorbed concentration which is classified into four levels according to defining characteristics ('nimitta').