Like any vacation, you'll want to ensure you're heading to a place you'll enjoy -- at least on some level. Even though you'll be spending a large chunk of time working on your volunteer projects, you'll still have some free time to explore the area.
Consider the local wildlife, flora, climate and temperature. If you hate rain, for instance, you probably don't want to work in a place where it rains every day. If you're especially scared of big bugs, you'll want to stay away from a rain forest [source: Tuttle]. Most volunteer vacations will offer some type of sight-seeing, so don't worry that you'll be stuck working the entire time and choose a place you've always wanted to visit, but perhaps couldn't afford. That doesn't mean you have to work in a rural environment -- there are plenty of volunteer positions available in urban ones as well. If you'd prefer to remain connected to the outside world, consider working in a metropolitan environment.
You'll also want to get a good understanding of the political climate of where you're heading. This won't be a big deal in a lot of cases, but you'll want to be aware of how a political system works regardless of whether there's a threat in the area.