I don't have much of a brain for expressing these things, especially tonight, but I'll give it a try. I'll include when Ne and Si compound Fi, because for me they often do. This is from the NP perspective, obviously.

Dominant Fi is:
- Being able to feel the same emotion (anger, sadness, anxiety, compassion) in a million different ways and all at once. No two moods (I wouldn't say 'feelings' as I always experience multiple and overlapping feelings at any given time) are the same. Some combinations of feelings can evoke previous moods (which can be powerful in conjunction with Si nostalgia for me), but they are never truly identical.
- When feelings don't just overlap, but are fluid, ephemeral and too complex to verbalize.
- Testing feelings for consistency and motives for integrity the way that Ti tests ideas for veracity and arguments for logic. Spending hours locked in your own head, examining your own train of thought, studying your subjective 'truth', symbolizing your internal and external life. (With my Ne, the possibilities for exploration become nearly endless, and my internal life becomes a tapestry of real events, reconstructed pasts, imagined futures and just plain fantasy).
- That feeling that nags you when you have had a thought or taken an action contrary to your values and the probing investigation that follows.
- The sense of deep satisfaction when you have done the 'right' thing. 'Right' can mean doing what is emotionally fulfilling (like pursuing a long sought dream) or ethically/morally correct according to internalized values.
- When moods are triggered by immediate environments, abstract ideas/ideals and mundane conversations. Everything that goes on around me is absorbed into Fi, which is affected depending on a number of factors. For instance, some smell that evokes a Si-based nostalgic memory can arouse Fi. The same goes for an idea that Ne finds intriguing and wants to play with.