GZA said:
I am generally not very good at, nor inclined to attempt, encapsulating linguistically the essense of a type, but I'll try to share a few of my impressions of INFPs based on my time at INFPgc and relationships developed there.
For full disclosure, I am in love with a man who types as INFP and I find his expression utterly beautiful and worthy of awe.

My observations are certainly in part based on my real life experience of his expression, but are also based in consideration of the broad range of INFP expression I've been exposed to.
For me, in guessing whether I'm in the presence of an INFP, I primarily tend to key off of a combination of introversion and hints of Fi judgement communicated in a particular way.
INFPs seem to express as perfectly capable of and willing to keep their own company, but also seem to have an empty space of longing for deep and intimate connection with the right long-term companionship. By my observation, INFP's choose their company carefully and would rather keep their own company than engage with unsatisfying company.
I usually observe the hints of Fi as an agitation with some expression in the external world that is violating an internal value. To my observation, INFPs are very adept at both a subtle and crafty calling out of people as well as direct expression of reprimand when there has been a violation of their values.
I get this 'dreamy' romantic somewhat serious vibe from INFP and similar to Nightning you get an impression and INFP is a sympaethic character and couln't hurt a fly. Gentle and friendly but shy?
Most INFPs I know I would consider as having a romantic heart, but I've seen it exhibited both without reserve and alternatively masked with a resigned romantic cynicism.
My experience of INFPs is quite often one of clever playfulness rather than seriousness.
I do see a "dreaminess" in many INFPs, but I don't exclusively experience them as lounging languidly on a cloud, contemplating the internal experience of life. Often I experience them as bounding with intellectual agility from cloud to cloud creating clever artistry. I've observed what seems a playful toying with the intellectual assumptions people hold, seeing what happens and how people react when everything is turned.
The emotional experience of others seems to penetrate deeply into an INFPs experience. I've seen this manifest in some of the most insightful and supportive caring I've ever witnessed or experienced. Though I've also seen more than one frustrated INFP accuse those around them of infecting their emotional environment with negative energy.
Most INFPs I have experienced do express as very gentle, yet I am also well aware they have sharp claws. Taking that gentle expression for granted might leave one suprised at the experience of finding them shredding something to bits.
I guess overall I experience someone as a possible INFP who generally keeps their own company, expresses desire for deep emotional connection, is intellectually playful and creative, takes strong (sometimes surprisingly strong) stands to defend their values, and acts on a deep sensitivity to both other's emotional experience and their own.