Open vs Closed Sets for studying topology Topologies can be defined either in terms of the closed sets or the open sets.  Yet most proofs, examples, problems, etc. in standard texts concern the open sets.  
I would think closed sets are easier and more intuitive for most people. So, is there a particular reason it is better to work primarily with the open sets?

Perhaps the reason is that many proofs and definitios rely on open neighbourhoods. I think that an open set that contains a point is more intuitive that a closed set that doesn't.
Take, for example, this definition of limit:
$\lim_{x\to a}f(x)=L$ if for every $\epsilon>0$ there exists some $\delta>0$ such that $|a-x|\ge\delta$ or $a=x$ whenever $|f(x)-f(a)|\ge\epsilon$.
