Hi,
I come from San Diego/Los Angeles California, and am also picky about my yoga classes. I went to so many classes in Buenos Aires that said they were yoga, but were really just stretching!
The best I've found is a studio where classes are lead by ex-pat woman who teach in Spanish and English. They are wonderful, and welcoming, and offer classes in San Telmo and Palermo. The classes are a mix of vinyasa and iyengar positions, and are pitched to a medium/moderate level of difficulty; beginners are welcome, and they also signal when more advanced students could go deeper into a posture or pose. (They'll also tailor the level of the class depending on the students that show up on a particular day.)
It's called Buena Onda yoga:
http://www.buenaondayoga.com/
~ Jenn