So at DrupalCon Austin I had a great time at the contribution sprints. I worked on some issues affecting Drupal.org, it was great fun!
The issues we worked on over the week range from simple things through to some pretty difficult issues.
Although Drupal core can always use more contributors, I would suggest that Drupal.org is also desperately short of contributors too.
One of the issues I worked on related to the tracker page for individual users, the bug-fix was already written, but we needed to write a test. Also, note that this issue was for Drupal core, not specific to Drupal.org.
So, I wrote the test that was needed, and then the patch got into Drupal 8 the same day! Turns out, if you focus on issues that lots of influential people have their eyes on, then you tend to get movement on your issues much more quickly. Unlike esoteric issues that tend to wither and die.
Anyway, once the issue was in Drupal 8, I backported the fix and test to Drupal 7, and it now awaits being committed to Drupal 7 but it's already been deployed to Drupal.org, cool!
I would suggest that if you want to contribute to Drupal, but aren't sure where to start, get yourself going with some Drupal.org issues!