1. fan if you like the person or their writing. If you resiprocate every fan u get, you'll wind up with a list of favorites you don't even care for. Also, don't expect other to reciprocate your fanniing, most won't unless they're new
2. YES reply to comments you recieve. Not only does it show the commenter you appreciate their view and comment, but also puts your hub in hubtivity which can bring in more HP traffic
3. A variety of factors--hubs published, individ. hub scores, community participation. All sorts of things. There are countless hubs devoted to this, just do a search. There are also forums out the wazoo about it