The first few times you go to stackexchange.com, you see the following:
What is this?
Stack Exchange is a network of free,
community-driven Q&A sites. We
highlight and aggregate the best
recent content from our entire network
here. Log in to create tag
sets to view questions on subjects
that interest you.
Wish we had Q&A on a different topic?
Help us create new Q&A sites through
the open, democratic process at Area
51. For more information, check
out the blog or read more about
But right now we don't really see stackexchange.com as the entrance point to the network. Most people will find out about Stack Exchange by visiting one of the actual Q&A sites, and only later realize that it's part of a larger network. Putting it another way, if I were telling someone about Stack Exchange, I wouldn't send them to stackexchange.com, I'd send them to Stack Overflow, or Gaming, or Cooking, etc.
StackExchange.com is more for power users of the network, to give them tools to manage all of the sites they're interested in with the Hot Questions page and Tag Sets, or track their placement in the Reputation Leagues.
Joel also wrote a very nice about page a few weeks ago.