Why does a string of arabic characters crash Safari and other web browsers?
As the arstechnia link to ycombinator page states, it is a "bug inside Apples CoreText font rendering framework".

This post on a Russian website drew attention to it:
http://habrahabr.ru/post/191654/

(translated page here: http://translate.google.com/translate?sl=auto&amp;tl=en&amp;js=n&amp;prev=_t&amp;hl=en&amp;ie=UTF-8&amp;u=http%3A%2F%2Fhabrahabr.ru%2Fpost%2F191654%2F )

The comments under the ycombinator article are the most interesting, with one reader pointing out that this bug is already fixed in Mavericks

Also, if I had enough reputation, I would downvote your question for putting an offending string in the question and thus risks causing a crash for some apple.stackexchange.com readers.  A slightly malicious or somewhat stupid move on your part, I dare say.