The main reason you should read Dreiser is because you won’t really like any of his characters.
That’s not at all to say you won’t feel for them or enjoy reading about them–you will, which is why I’ve started with the (perhaps rhetorically obnoxious) provocative statement. Besides having oodles of delicious moral ambiguity, Dreiser’s books are populated by characters are decidedly not-stock (unstock? antistock? stock-taneously combusted?). There’s not any black and white in the palate to mix and get gray.
All of the characters in his novels do things you won’t like. Yet you’ll understand why they do them. Take Carrie. She wants to learn how to be a refined, cosmopolitan lady who exudes the kind of grandeur she sees in other women about town. She trades in her country charm and earnest naivete for affectation and imitation. She becomes a fancy carbon copy, practicing faces in the mirror, worrying constantly about the image she cuts, mimicking the mannerisms and gestures of all the grande dames she admires. It’s awful to read about. Yet you’ve just read about how underestimated and disrespected she had been, and now people are starting to notice her and treat her like a human being. Wouldn’t you do the same? Then you learn that she is a born actress, and all this imitation comes naturally to her because her calling is the theater. The primping and preening still seems obnoxious, but it’s all forgivable now. It’s as if her life is a dress rehearsal for playing other characters.
Then, when she is practically kidnapped by her overzealous, possessive lover who won’t take no for an answer, instead of giving him a black eye and and hightailing it as far away from him as possible, she feels sorry for him and marries him. And you’re disgusted with her again. But you know she’s trapped, so…
Speaking of the overzealous possessive lover, Hurstwood is the same way. There is so many things about him to hate. Stealing. Treating Carrie as a amusing little possession. Cheating on his wife. Leaving his wife and kids without a word. Lying. Lack of motivation in getting a job. Gambling all his money away instead of making sure he and Carrie were taken care of. Yet, when he looks for work (less often than you think he should, but still, he looks), you understand his problem. He has no money, but he has the air of wealth still about him. No one will hire him for menial tasks until it’s too late–eventually, he becomes such a wreck he can’t get any kind of job at all. He’s so pathetic, but his difficulties are so empathetically recounted and his memories of success and happiness are so bittersweet that you can’t help but understand why it’s so hard for him.
I love this. The way Dreiser crafts his characters is so authentic, because so often I feel this way about real people (in essence, “I hate what you’re doing, but I understand it.”) Human beings ARE this complicated, and I think Sister Carrie is such a beautiful testament to Dreiser’s brilliance. With great finesse, he really conveys human complexity. The Genius also does this (a book which is free, thanks to the wonderful folks at gutenberg.org–it’s long, but SO, SO worthy of your investment of time. I liked it better than Sister Carrie.) The main character in The Genius is another example of someone who does terrible, terrible things, and yet you understand him. You even cheer for him as he’s wrecking other people’s lives around him. It’s amazing.
Another thing that makes Dreiser so interesting is that his books just contain snippets of people’s lives. It’s like if someone’s life story was film on a reel, and Dreiser takes a scissors and makes two casual, random cuts. Poof, there’s the plot he works with. Then, through magical, crafty, authorly powers that I can’t imitate, only recognize, he finds a way for the sequence of events to seem rounded out when it comes to an end. For Sister Carrie, he ended with a kind of life lesson that could be applied to all the characters. In The Genius, he leaves you with the hopes that Eugene has finally grown up, at least a little. But the actual plot really has no rhyme or reason to its beginning and end–it’s as if Dreiser is simply handing you a snapshot and letting you examine it close up to feel what the moment in time was like.
In Litero: An Evaluation of Literature 
The least interesting characters are the ones you’re sure whether to root for or against. You’ll never think further on them, never relate them to people in your life, and certainly never really talk about them much with others. It’s the multi-faceted, morally ambiguous ones that make people talk.
I think this transcends books and is apparent in pop culture phenomena as well. The most memorable characters in movies/shows are like this too: Jack Nicholson’s Colonel Jessip or Charles Foster Kane. People *debate* and form strong opinions about these characters, whereas with most cookie-cutter pro/antagonist types the discussion is more like “did you seem how many things {insert Stalone/Gibson/Willis character here} blew up while saving the world!?!”
I love the connection to pop culture–there’s definitely been an uptake in moral ambiguity as it concerns TV and movies. But I think ambiguity, like pure evil/ pure good, can be done in a way that is exasperating. Law and Order, though pretty reliably good, sometimes gets predictable and tired in its moral ambiguity. On the other hand, every once in awhile, pure evil/ pure good will be done so right that it gives you chills (see Edward Norton in Primal Fear). I think when either ambiguity or an archetype is done well, it leaves an impression. This leads other, less talented folks to try to copy it in cheap ways, which in turn leads to a lot of yawns and sighs.