The community, surprisingly enough. Although I don't feel that there's as strong a sense of community any more, especially over in the video games forums which is where I probably spend most of my time. The regular users I remember are less frequent visitors now and their posts are becoming fewer and further between. It's a shame.
Every once in a blue moon I get talking to someone via these forums that I get on really well with for one reason or another. Had the pleasure to meet a few of them too. I guess that's partially why I stay here.
I guess the main reason that I stay on this site is because Newgrounds has essentially become the centre of the internet to me. Newgrounds practically is the internet as far as I'm concerned. Everything else is just an added on extra. It would be hard for me to leave this place because it's probably where I spend the most time while I'm online.