It depends on how your uni goes about marking it, really. It's often in relation to how the other students did too.
Also, depending on your subject, I think it's a lot to do with "natural" flair. Obviously you have to put a lot of work in as well, but, frankly, if you're not a natural you probably won't get a 1st. You've also gotta take into consideration the law of diminishing returns. Sometimes it simply might not be worth slogging yourself for a first!
I guess it's good to go into uni with the idea that you're shooting for a first, but don't kill yourself over it
<edit> oh yeah, and if you're doing an arts subject, there's plenty of books out written by lecturers telling you what's needed to get a first.
We have the answer...