If you know you'll be on reasonably good roads and resupply is relatively easy a lightweight tourer is great. Serious adventurers have made them work on round the world and expedition trips, but for mortals it's probably best to use a bike with more carrying capability and wider tires the farther you get away from civilization.

But if you are thinking of riding across Europe a lightweight tourer would be great and you can pick from any number of steel or carbon relaxed geometry bikes as long as you figure out how to carry your gear without having to resort to massive panniers. So I'd figure out your gear first.

As an example I've gone from a steel sport tourer to a Cervelo RS by incrementally modifying my gear and i far prefer the ride of the Cervelo, particularly when climbing and descending. Search on lightweight or ultralight and you'll come up with lots of examples of people's setups and trips. There's a particularly good discussion here:

http://www.crazyguyonabike.com/doc/?doc_id=9738

This summer I rode the Cervelo RS about 550 miles from Boston to Buffalo and here is my setup.

http://wheelsofchance.wordpress.com/.../25/gear-list/