We've been doing this for three years now. This couple seemed to focus on having a year abroad, we've made it our lifestyle, and we're doing a startup (rather than consulting). I started this as a response to the "how to keep productive" question, but I'll try to address the other questions people have been asking in the thread later on. We also have the issue of taking some period of time to get back into the produc…
Here are some warnings, learned through experience:
As a foreigner you will often be treated as a second class citizen. That may or may not matter much, depending on where you go, what you do, whether you speak the language, and how many quality contacts you have in your host country.
I found it disheartening but eye opening just how poorly I was treated sometimes just because I wasn't a native, didn't speak the language, or was just being a respectful tourist.
Dealing with the bureaucracy in foreign countries when you're not a native could be a nightmare. It's even worse when you don't speak the language.
Again, speaking the language is also really important if you want to have natives as friends. Far too often expats socialize within a cocoon of other expats. And it's very easy to become very isolated in a foreign land, especially if you're not naturally very social and outgoing.
Flying and various other mechanics of travel might seem fun when you first start, but they'll become routine and maybe even onerous before too long -- especially with the ever increasing hassles of going through security and customs.
As a native in your own country you can frequently get away with a lot of things and talk your way out of many situations that you can't when you're a foreigner.
Foreigners and tourists often get taken advantage of because they don't know the rules, because they're afraid of getting kicked out of the country, and because they get little sympathy from some natives who look at them as nothing more than walking cash machines.
Of course, sometimes foreigners can get away with stuff that natives can't.. especially in the realm of social faux-pas, where a foreigner can be forgiven for being ignorant of local customs. But even here, not everyone is equally tolerant.
That said, I've benefited greatly from travel. It's been a very educational and mind-opening experience. Through it I've met many wonderful people and seen and experienced things I could not have imagined had I stayed in my home country my whole life.
I definitely encourage everyone to travel. But doing so long-term takes a special kind of person who's able to put up with a lot of inconvenience, hassle, and insecurity that you just wouldn't need to deal with if you'd just stayed home. (Of course this last bit wouldn't apply if "home" is some war-torn or economically depressed place compared to the countries you're traveling to.)