Just back from BA and I was so misled as to how to dress. Here's the rundown, men:
If you're under 40, you'll blend in wearing skinny jeans, yoga pants or fitted sweats and athletic shoes. The "hipster" look is popular, with flannel shirts and Chelsea boots. For casual evenings and most restaurants, a polo shirt and jeans that aren't trashed are fine. Basically, Portenos dress like Americans. Also, unless you're shopping for a locally made designer leather jacket, there's nothing to buy in BA. It's all the same stuff you'll find here in the U.S., but it's more expensive.
I was a little surprised to see so many women in BA wearing yoga pants with really ugly 4" black platform sandals and awful t-shirts with English slogans.