Oomis is right. In the 60s and 70s YI was very modern orthodox. Today, many of the YIs out of the metro NYC area have mechitza that would be just as much at home in an Agudah shul.
The big difference is, the socializing that goes on outside of davening and shiurim. Its not uncommon to have a mixed gender Laag Baomer picnic or Chol Hamoed trip in a YI setting (something that would raise a hue and cry in a black-hat crowd)
Plus, many of the YIs still have the dual flags up by the Aron Kodesh (and sing Anim Zemiros in that unmistakable tune!)