I have not done the research to figure out if these are unique to Argentina, or if some or all are common in other parts of Latin America. Probably.
Anyways, here goes a list that will not include all the usual tango, maté, soccer, gaucho, asado, plastic surgery, blah blah blah.
1. Instead of knocking or ringing the (often non-existent) doorbell, people stand outside and clap loudly a few times. They don't call out hellooo? or its Spanish equivalent. This is only really true outside of the big cities (it obviously doesn't work very well with apartment buildings).
2. If you want to sell your car, you put a bottle of water on the roof of the car whenever you park it anywhere. If anyone is interested, they can come and clap for you. Why not a 'for sale' sign? Well, why not a bottle of water on the roof?
3. Bidets are standard bathroom gear here. When I say standard, I mean that even gas station bathrooms have them. In addition to their obvious uses, they are really handy washers of dirty little feet, and pretty wonderful toddler-sized sinks (and fountains).
4. The 'come here' gesture is a more contained version of my 'go away' gesture: palm facing outward and then flopping forward one or more times. I know this is true for all of Latin America, along with a tendency to hiss or go 'tchh, tchh' instead of saying 'hey' to get someone's attention.
5. The friendly or polite way to acknowledge an unknown passerby is not with my standard, toothless almost-smile, to then quickly look away. Instead, you mutter some kind of greeting - hello or good afternoon or bye - and then look away, or not. (I still have to consciously remind myself of this one all the time. I often find myself doing my almost-smile thingy, and then they often say hello, and then it seems like I wasn't going to say it, because they already caught me looking at them and sort-of-smiling and looking away.)
6. It is very, very close to impossible to find a mini kid-sized backpack that does not have one of the top 10 commercial characters plastered all over it (barbie, ben10, disney stuff and whatever else). Needless to say, all divided between pink for the girls and anything else for the boys. Same goes for toothbrushes, stickers, pencil cases and more. This means 3 things: I am still looking for a backpack for the monkey; I must stop judging all the parents who buy such crap for their kids; and entering shops often puts me in a really bad mood.