Colour.
A little strange I know but colour can add a lot to an environment, or tone. Darker colours pertain a darker mood, greens for something lush, red for anger/lust, browns for decay. Colour can be used in a lot of ways, and helps 'paint the picture' so to speak, to the reader.
I'm also one of the writers who just writes for fun. I see no career in it for me, I just like to put my stories down and share them across the internet. Unfortunately I keep breaking one of your rules. Dedication. Between work and real life drama I don't always get my fingers to the keyboard or pen to paper. However, I do enjoy writing, have a story running in my blog at the moment, and some others at other sites. I always write by the seat of my pants. I have a basic idea, then fly with it. It's worked so far. With my current blog story, I completely changed the setting and one of the main characters when I just sat down and looked at it. It was fun.
Also J.K. Rowling became so popular partially due to controversy. The Catholics didn't like the way she portrayed witches and wizards as good guys, and that riled a lot of people up. Especially considering that the book was initially meant for children (albiet the series got a littler darker along the way... still meant for young readers though). People hear about this controversy and want to know what its all about. That's how it got so popular. Same thing happened with Dan Brown's DaVinci Code. Controversy, people wanted to know what was going on. There are quite a few examples of that happening.
As for violence, and sex, and all that fun taboo stuff, like FBI has been saying, write it in context. If its just thrown in for shits and giggles, then it doesn't fit the mood. I write a fair bit of violence into my stories, but I see the world as a violent place and so are my own little worlds because of that. A lot of my characters swear a fair bit, I actually cut down the swearing in my writing from what I hear people I hear on the street, at work etc. using in regular conversation.