The community here is the best. If not for the community, I wouldn't still be here.
Besides that, I also like the Hub Tool. It allows you so much flexibility to create great hubs. I also like the Learning Center. There's a lot of helpful, legitimate advice there that can help you here on HubPages or on other sites too.
I also like that HubPages is taking steps to provide really great content and improve the user experience. It's kinda frustrating for older hubbers who are used to things being a certain way, but in the end, I think it's necessary.
I've tried Squidoo, Bubblews, SeekSuccess, and InfoBarrel. They all have their pros and cons, but to me HP is the best overall. At least as far as user-generated content sites are concerned.