I left my job when I was in my late 20's to make a year-long round-the-world trip. Leading up to it, I was up for a promotion at work and my managers and colleagues were very skeptical about my decision to leave at such a time. I'm so glad I stuck my guns. It was easily one of the best decisions I ever made.
My partner, who I had recently started dating at the time, was unhappy with her job and also decided to take the leap. In 2011, we spent 313 days traveling around the world together. Ten years later, we're still together!
As for trip logistics, very little was planned in advance. We both gave up our apartments and put our belongings in storage with friends and family. We started the trip with one-way tickets from the USA to Asuncion, Paraguay. Not a common first destination on a RTW trip, but we had used miles and the cost was the same price to go to a relatively obscure place rather than a more common one. Paraguay is a fun one if you are vegan/vegetarian, but that's another story.
Prior to the trip I had stockpiled a decent amount of frequent flyer miles. The late 2000's were crazy with respect to mile program promotions, but that's a whole different story! Those miles helped offset most of the long-haul travel costs but on the ground we preferred to travel as cheaply as possible with buses, trains, etc. The main thing the miles enabled was access more remote and uncommon destinations. As an example, one of our travel days took us from Mongolia to Palau. I don't think that's a common origin-destination pair.
I will second what others have posted here about moving slow. The places we have the fondest memories of are the ones where we spent the most time. This cuts both ways though. When you are traveling/living this lifestyle it can take some effort to find that magical mix of affordable housing, good food, and interesting activities. When all those factors align, it's easy to "get stuck" and then later have to cut things from the larger itinerary. Burn out from being on the road is another real factor. We felt it strongly at about the 6 month mark, which, conveniently was timing with a detour back home for my brother's wedding. After that we were eager to get back on the road.
Regarding costs, we found the trip to be quite affordable. Our burn rate over the year averaged out to 1000 USD per month per person, however, we had intentionally avoided the more expensive parts of the world (e.g., Europe, OZ/NZ, etc.). Aside from a few splurges, we chose to stay in budget hotels/guesthouses wherever possible. We had also offset much of our long-haul travel costs with airline points/miles.
I would love to take a trip like this again and I'm glad I didn't save this experience for retirement. I can't speak highly enough of doing something like this if you have the means.
If anyone is interested in more details about the trip, I kept a blog here:
http://www.roamingryan.com/