You get to see who's in it before it starts? We don't on UK Big Brother. The contestants are hidden away from their family and friends for 2 weeks, so none of them even know that they're going to be on it. They're only introduced when they enter the house.
I watched American Big Brother one year, and it's quite different to ours. Like your contestants are allowed to discuss nominations and plot to vote each other out. Ours aren't allowed to do that. Yours is more bitchy and tactical. Ours is just bitchy!
Also, if I remember rightly, your version doesn't have a public vote? So I suppose being 'nice' doesn't count for much, as they don't need to get the viewers to like them. Here, the weekly eviction is decided by the viewers (although the two housemates nominated for evicton is decided earlier in the week by the housemates), so if you're really annoying, you don't stand a chance.
Personally I can't stand Big Brother anymore. It was good the first few years when they had NORMAL people in there and you could watch the way they interacted and formed relationships. Now the only people who go in are fame hungry attention seekers. I watched a bit of the first episode of the UK version this year and I didn't see anyone who seemed remotely likeable.