I'm well behind the curve on this, as I see it was announced at the end of February, but yesterday I noticed a significant user interaction improvement on Flickr around adding photos to groups you don't currently belong to.
The old way was a bit cumbersome. Someone would spot your photo of the lesser-spotted outer Mongolian tortoise-shell widget, and you'd get a Flickrmail saying:
Hi, out of your 6,512 photographs, I notice that once, in the corner of one, you captured a lesser-spotted outer Mongolian tortoise-shell widget. I'm in a group fascinated by the lesser-spotted outer Mongolian tortoise-shell widget, and we'd love you to join the group and submit your photo, and thereafter be bombarded forever more by pictures and inane chat about the lesser-spotted outer Mongolian tortoise-shell widget
The answer, as in the above screen-shot inviting me to join a QPR fans group, was always a 'no' from me. Why would I join a QPR group? I'm a Leeds fan who took some pictures at a QPR - Leeds match. I've no interest in seeing any other pictures of QPR, or chatting to any QPR fans, nice though, as I'm sure they are.
Flickr, though, have now introduced a way to submit pictures to a group without having to join it. It is such a simple change on the front-end, but brings back the joy of sharing.
Now, someone simply asks you to submit a photo to their group. You get asked not via Flickrmail, but via your recent activity page, the whole thing takes one button press, and you're done. The group gets the photo, and you never have to worry about the lesser-spotted outer Mongolian tortoise-shell widget again.
I'm still getting people nagging me to add photos to groups the old fashioned way though...