Its not nasty unless you go looking for nasty images.
People post photos and follow friends, or people they admire/interetsted in to see what photos they have posted. Theres an opportunity to add comment but ther is rarely much dialogue.
The instagram account can be private, so only visible to accepted followers, or fully public. Quite often people who use Twitter use it to post and share images.
I'm not so convinced its attractive to mainly young people. Its more about people who prefer a visual form of communication rather than chat. Although people do comment on the images posted, its less chit chatty, and slightly less intrusive than Facebook.