I avoid CrowdFlower like the plague. I hate that company. Too much work on each task and way too little pay.
They also hire out through other sites besides mturk, no better pay. I was kinda hoping to at least see it higher somewhere. They're easy jobs but not worth the time.
They're called "gold" questions (or answers). They intermingle some questions that they already know the answers to, and if you get them wrong, you are called out. A lot of requesters do this to weed out scammers. As far as their business practices, I think CrowdFlower, aka Dolores Labs, is disgraceful. They fully exploit workers and explain in the media that workers do their HITs because they are fun. Yeah, right. Even when they do something that seems to pay "enough", they limit the HIT so that you can't make any real money off of it. And yes, I've heard instances of them rejecting. This company wants to be the poster boy for crowd-sourcing, but is actually just bad news.
Yeah Whimsy you are right. Just now me seen in a site their works there and there I see many people there as worker is not ready to their HITs for their very very low pay. Also when I tried to earn through a facebook app it too come with CF tasks.