I've wondered the same thing about the United States. We've seen a LOT of our own country (more so than any other country - I feel quite well versed in ~90%!o(MISSING)f it) and our blogs reflect this. We met dozens of other international travelers camping, hiking, seeing the sites - but very few other Americans.
Several times we'd ask locals in town how to get from a to b, or what the scenery was like at globally renowned destinations (a la Grand Canyon) and people would say, "Oh I don't know. I've never been" even though they lived about 15 minutes away. Crazy!
I am adamant that traveling doesn't necessarily mean crossing a border, but I think the reason's people don't explore their home as much is two-fold:
a) Places like Australia, Canada, the US are soo large - for the distance traversed you could easily 'hit' dozens of countries in a different part of the world. If you base your "traveler boasting experience" on passport stamps or country counts, you're going to spend a lot of time seeing amazing things, but you won't rack up the numbers
b) it's not "exotic" or "ethnic" enough (language is the same, freeways are the same, Walmart is the same) and because of that people assume if they've seen one part, they've seen it all. I mean, that may be true if you don't deviate from the freeways, but ecologically and culturally any huge area of land will be diverse - the U.S certainly is. You have to try (although not that hard, really) to find experiences that "impress" people, because I can't go outside, snap a photo of any random street and have people say "Ahh... it's so different
! I wish I could do/see that"
So I guess in sum, domestic travels seem to be less brag-worthy. I don't necessarily agree with this, but that is the general attitude I come across.
I thought cost might be a component (and to those who choose S.E Asia or S. America this may be true) but Europe and Australia are main destinations for American travelers, and it is both more expensive to get there and more expensive to travel around once you arrive then it would be here, so not sure about that.
As for my favorite part? I love the Pacific Northwest - fresh seafood, evergreens, snow-capped mountain peaks, hiking, local fruits and agriculture, boats and waterways, and even a temperate rainforest all wrapped around an urban city that embraces the outdoors as much as the outdoors tries to encroach upon it - I love this region of Canada too - B.C/Vancouver, although since it's in metric it gets to feel foreign 😉
And the National Park system - Yellowstone, Grand Canyon, even (believe it or not) the parks of South Dakota. I couldn't live there, but it's a very interesting place to visit.
[Edited: 2012 Mar 19 13:43 - Stephanie and Andras:35953 ]