I recently replaced my ancient Jansport travel backpack, which had circled the globe with me three times. I got something smaller, figuring it would force me to pack light and sensibly.
I settled on this sleek (as backpacks go) Eagle Creek one, which supposedly even fits as a carry-on bag if you want to push it. Since mine is always ready to burst at the seams, no matter what size I use, I just check it. This is a very sturdy, well-made pack that can take anything you throw at it.
It's small, so maybe not so good for a round-the-world trip unless you are staying in hot climates the whole time. On the other hand, this is one of the few packs where you can walk around with the day pack attached and not feel like you're going to topple backwards at any minute. Very comfortable to wear around and the size keeps it light. Lots of useful straps and the suitcase-style zipper make it very easy to pack and unpack. I'm sure I'll still be using this five years from now.
See more about the Eagle Creek Continental Journey pack at Backcountry.com.