As an urban greening (and livability) advocate, certain things stood out: the canopy of trees over almost every street; the wide, green balconies; the excellent urban transport system; and a street design that fosters a lively street culture.
Despite all the financial nightmares that this country and city have been through, the trees are maintained, and I noted a lot of new tree planting, as well.
I've noticed a trend in many cities (Bogotá, for one) to exclude balconies from new buildings, presumably to maximize indoor floor space. This trend doesn't seem to have caught on in Buenos Aires. Even the newest apartment towers I saw include broad balconies.