What exactly made him successful

Yeah but without a producer the director can't make the movie or at least i'm pretty sure that's how it works.

Yeah but the studio is responsible for funding it. Think of him like a bank. How many people are capable of buying a house without a mortgage? People want to buy a house the same way the director wants to make a movie. But neither of them have the money to do it. That's where the bank/studio comes in.

The only reason Harvey was so successful is because he took chances on independent films and allowed directors more creative freedom. Where typical big league studios want to have more control over how you make the movie they're funding, Harvey was always more relaxed about letting directors maintain their vision. That's why Tarantino and Kevin Smith have maintained a steady relationship with him. I don't think Warner Bros. or Universal would allow them the same creative freedom Weinstein did.

Surprisingly (considering his behavior), he helped produce the classiest films in Hollywood, with very very very few aimed at a commercial/male teen/cop-out audience.

THE THIN BLUE LINE
SEX, LIES & VIDEOTAPE
THE CRYING GAME
FLIRTING WITH DISASTER
PULP FICTION
TRUE ROMANCE
THE ENGLISH PATIENT
GOOD WILL HUNTING
KILL BILL
EMMA
SHAKESPEARE IN LOVE
GANGS OF NEW YORK
JANE EYRE
THE OTHERS
THE AVIATOR
CHICAGO
INGLORIOUS BASTARDS
THE KING'S SPEECH
THE ARTIST
THE BUTLER
MY WEEK WITH MARILYN
WOMAN IN GOLD.
The LORD OF THE RINGS series

He produced GREAT films that just about any artist would be thrilled to have on their resume.

(He also did the occasional horror film like SCREAM, but at least they were intelligently done.)
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Persistence and a bold, manipulative salesmanship type of personality required to convince people to take risks. That's his only real "talent" - raising the capital needed to fund film production. Sadly, this is the key to rising to the top in almost any profit driven corporation in America. And it is because of these same traits that he was able to get away with what he was doing for so long. He's the classic emperor with no clothes.