I don't even feel that it is necessary for success - whenever discussions like this come along, people will say things like "you make your own luck" or "luck provides opportunities, hard work takes advantage of them", and I agree - to a point. That point is usually far less than others would say.
Using myself as an example, I don't feel right saying that I'm a hard worker. Sure, I wasn't at the lowest end of possible effort, but I'm far from the high end as well. Figuring out if I'm above/below the 50% mark requires a lot more understanding of what is actually normal than I have, but I believe I'm close to average, probably a bit below.
Despite this, I've had a ton of luck and success. And when I look at people that are more successful (wealthy, at least), some of them have worked harder...and others have had just more luck.
I had a great high school which let me coast through college. Because I didn't have the GPA to get the degree I wanted (restricted admissions), I ended up with a "make-your-own" degree. (Which was good because I had accrued so many electives out of general curiosity that I'd have been in college for years extra to qualify for any other degree). I had a 75% tuition discount, which meant that even though I could afford student loans, I didn't need any.
I fell into a profession just as it was becoming hugely in demand. I don't program because it makes big bucks, I program because I've always wanted to. It just happened that as I entered college the Web went mainstream.
My first job out of college was working on coldfusion (which I didn't know) on an NT environment (which I hated), all because I had fired off an email telling them I could show them how to use Perl on Linux, and they had just switched to Oracle on Linux (unbeknowst to me) and wanted someone with some Linux experience. You could argue that that email was "work" on my part, but really it was just a punch-drunk sarcastic email sent after a couple of hours of putting out legitimate applications. Total luck.
3 months later I got a contracting job for the state at double the salary (3 months post-college experience apparently counts for a lot). It was a 6 week contract that I stayed at for 7 years (at that point I got hired full time and stayed for another 5 years). The job was easy and I learned a ton because they had no real work for me, so I just did what I wanted. Work? Some, but mostly just luck. Certainly a ton more luck than other people that were working harder.
It goes on and on like this. And I'm not as lucky as some: I could have been born into a rich family, where I'd have to work hard NOT to make money. (Heck, we've seen that someone can have multiple failing multi-million dollar enterprises and still roughly match the market).
I'm not saying I didn't do ANY work...I'm saying that my success (and the success of many others) owes far MORE to luck than effort. In fact, I'd go so far as to say the majority of people I know that are successful are so because of luck. Many of them work harder than I do...but not harder than other people that are less successful. I don't think that's because the less successful people have had BAD luck (though some have), I think it's because the successful people have had more luck than the less successful people, by and large.
Yet in discussions like this people always want to emphasize the effort. I don't know if that's because my judgement is wrong, or because people are really uncomfortable facing the prospect that their effort counts for so little in the wider scheme of things.