My answer extends on that of @gentmatt, which in turn extends on the answers from @stuffe and @NathanGreenstein.
I'm impressed with this community, and how we didn't let crazy incentives destroy our sense of community. The promotion wasn't designed as a competition, and it didn't get competitive until people blew the thresholds out of the water to a degree unanticipated in the design of the promotion. I think we did a good job staying non-evil, but, of course, incentives do matter, so perhaps we can tweak them in any future promotions
How could we have done this had we known then what we know now?
In hindsight, I think it turns out that the winning bars should have been higher in terms of quality (more posts with at least 6 or even more votes), perhaps lower in terms of quantity (encouraging a lot of two vote questions/answers maybe isn't a great thing for the site), and shorter in terms of the length of time the promotion runs.
If the winning bar for this one were so high that only one crazy person were likely to be able to make it there, I suspect it wouldn't have been a competition; it would have been a lot of people cheering him on. But then the promotion wouldn't have drawn in as many people.
I think the idea of some fixed prizes and some randomized is a good thing, but I'd like to see a different mix, with a smoother gradient between the deterministic portion (you will win this prize) and the purely stochastic (anyone who meets the minimum threshold has an equal chance). No abrupt cutoffs mean no cutthroat competition to be the one right above, and not below, the cutoff, but improved odds for better performance encourages everyone to do their best.
Perhaps something like
- the high scorer in the promotion wins an iPad.
- for people who complete x shares and vote at least y times, each question or answer with 8 (10?) or more votes gives you ten tickets in a drawing for one of 5 iPads.
- for people who complete w shares and vote at least z times, each question or answer with 4 (5?) or more votes gives you ten tickets in a drawing for one of 25 iPod Touch devices.
- No one can win more than one prize.
- If fewer than 5/25 people meet some minimum threshold, fewer prizes are given out.
Even better if the random drawing algorithm could be sophisticated enough that if you cast upvote #1-8 for a question or answer that ends up reaching 8 votes, it slightly improves your chances of winning yourself (things that would give you ten tickets give you 11 tickets, for instance) -- wouldn't it be great if we could reward constructive edits and constructive voting, given that they're what makes the site work?
Then, there's still an incentive to vote for quality. There's an incentive to post good stuff, and not just anything. And there's a link between merit and success, but some element of chance also.
It's not just luck, but it isn't cutthroat competition, either. Helping along your fellow participants is rewarded, but there's a direct link between posting great things and improving your own chance of winning.