My wife and I traveled for a year back in 2008.
Most of the trip is documented here: http://honeysun.wordpress.com/
We spent 4 months in Africa, 7 months in Asia, and 1 month in some Pacific islands. One thing we learned is that while a year is a long time to travel, damn, it's a huge world out there; you're gonna have to make tough choices.
For us the hardest part about planning the trip was finding someone to take care of our two cats (thanks T & S!) and finding someone to rent our condo. I left my tech job, and my company was really cool about it. My wife was between college and her first full-time job, so it was perfect timing for her.
I recommend looking at round-the-world tickets. Various airline alliances offer them. For a fixed price you can get up to so-many flights and so-many miles. It worked out well for us. We had about 14 flights taking us over 30,000 miles and it was something like $5,000 pp. These tickets are complex to book. Each has different rules. Our tickets required us to keep traveling one direction (we chose East) and to pick as many of our flights in advance as possible (airlines don't actually know their schedules more than 11 months in advance). So we knew where we would fly out-of/into every few weeks, and in between we had free reign, which was great.
I agree with the other folks on this thread - you can't really plan a full year trip the way you do a regular vacation, nor should you. Part of the fun is to wake up each day and say "what do I feel like doing today?" We ended up in entire countries that we didn't plan to visit (Uganda, Rwanda, Myanmar...), simply because we met people along the way who said "you gotta go there!"
Two things I do suggest looking at in advance:
- Weather. We planned everything around being in Nepal around spring-time, which is one of the better times to go trekking.
- Festivals. By dumb luck we ended up in Lalibela during their Christmas celebrations and it was one of the highlights of our trip - it felt like walking into Biblical times. On the other hand, there were other events that we just barely missed. So, for the next trip we'll definitely check out the big events happening in different countries.
I agree that you can get a lot of good advice from other backpackers, especially in countries that have a real backpacker/hostel scene (like in southeast Asia). We also had good experiences with Couch Surfing.
In retrospect, my wife and I wish we didn't rush so much. We rarely stayed in any city more than 4 days. We were far too anxious to see as much as possible because "when are we going to be here again?" But that's a slippery slope. After about 8 months we hit a low point where we were just exhausted from traveling. I started missing creature comforts from home. I just wanted to go to a freakin movie theater, and to use my Sonicare toothbrush. I know, totally first world problems :-) Anyhow, we took it easy for a couple of weeks and bounced back. But now when we talk about the next big trip we'll do one day, it's more of "let's live somewhere for a few months and take short trips nearby, and maybe learn the language too".
Since returning from the trip, my wife applied to med school. We had a lot of debates about how much she should/shouldn't talk about the world trip in her applications - it's hard to tell how people will react. She ended up including it in her applications but not too prominently, and she did get accepted (woohoo!!).