For me it was the UI / commenting system. I had been a lurker on slashdot for years but never made a comment. Then I went to reddit and was commenting 5 times a day because it was so light weight and easy and was totally pseudononymous. (re: the last point - I have not made one single public comment on Google+, while I have made thousands on reddit. Reddit has just the right balance of pseudonymity). I also think that making it possible to edit or delete your comment after writing it, which was quite radical at the time, improved the average quality of conversations enormously. The very few poor quality posts I made were either voted to oblivion, edited to improve them within 5 minutes or deleted if I realized I should never have opened my mouth.