Hmm ... I have wondered about the same thing for some time now (this is in response to the OP and his general ideas and questions)....
I would definitely agree that slice-of-life as a genre is largely (but not solely) based on mood or ambiance. Maybe even the "label" of slice of life could be where that ambiance or mood or setting is the "main character" of sorts in the story?
But it is not just that - it seems that the slice-of-life genre is most often labelled at those aspects of life that are quietly positive, provoke reflection and (very often) gratefulness for the "simple" things in life that we "take for granted" ... perhaps a retaining of the best aspects of being child-like: things that are easily forgotten and regarded as "non important".
It's almost a whole presentation or perspective combined with a reminder on a certain of spectrum of attitudes and dispositions that the producers of the art form (whether manga or anime or whatever) feel are lacking. In many ways it is reminds me of a certain type of 19th century European Romanticism in the arts such as that promoted by the British poet Wordsworth - things like simplicity and spontaneity (associated primarily with the sort found in children, again).
With this in mind series that have most often been associated with "slice of life" come more into focus for me: things like the Aria series, the Tamayura series, the Hidamari Sketch series, the Yokohama Kaidashi Kikou manga, Sketchbook Full Colors, etc.
There are plenty of other series, though, where "slice of life" qualities are present and maybe even stand near the "primary elements" of the series: things like Acchi Kocchi, Lucky Star, Miname-Ke and such - although for me the "main elements" in these stories is comedic.
Coming-of-Age, Comedy and Drama (which seem the most common elements associated with those series to which the slice of life term is most commonly associated) are all, of course, elements of "life" ... and life in the sense of the sort that the slice-of-life most generally focuses on (as opposed to the "nitty gritty of reality" or the "hard boiled realism" that for some people is "real life" much more so than some sort of "vague emotionalism hankering after a period in ones life that is impossible to return to" or some such).
Yet I guess for me the main focus is the question - some examples: Aria is slice of life primarily, but has sci fi elements and coming of age elements as well. Miname Ke is comedy primarily, but has slice of life and coming of age elements. In Aria the world, mood, backdrop and ambiance is the main focus (and even "character" in a way), whereas in Miname-Ke it is comedy that holds the primary place.
In this sense slice-of-life is a much rarer creature than people take it to be - and I guess of the three recent anime series that ended in summer 2012 that were mentioned in the OP I would guess (just my opinion, of course) that Hyouka is nearer than the other two to the slice of life genre - but I am not sure that I would call the main character and mood of Hyouka to be slice of life per se ... although of the three it certainly painted the strongest setting and background and the like: Kokoro Connect was much more of a drama focus for me personally, and Tari Tari was more of a Coming of Age.
At least for me.
