Ok, maybe I was bias with title, sue me.
When I got back to the US, I tried to keep the explanation of my experiences brief. I would simply tell people that I loved my time there, I would go back in a heart beat, and I would encourage the most seasoned traveler to the most novice to check out the country.
For you novice adventures, USAC is a wonderful program. They're going to get you the necessities- housing, classes, quick tips about the city, and they even throw in a few group adventures for those who like their trips planned out for them so they can relax.
If you're nervous about the language barrier, don't be. USAC offers beginner courses. When I first got there I didn't speak any Spanish, y ahora puedo hablar español un poquito. Do be warned that a lot of the locals (especially the university aged students) speak a fair amount of English, you will have to be persistant at times if you want to practice your Spanish.
Host families will very from student to student. Usually their rules are pretty basic. Respect their house, don't be noisy when you come home, and don't do anything you wouldn't do in your grandmothers house. Some folks will build a great relationship with there families, but don't think your family wants to take up all of your time. They often will invite you to events and try and help you experience the culture, but if you would like to make other plans they are very supportive of that. Sometimes there is a student or two who will have an issue. USAC does a great job to mediate any issues or help you change host families as needed.
When it comes to the people in your program it really varies. I was not a big fan of my first semester group (although there were a few fantastic people), but when Spring 14' came around we were like a big family. (oddly enough, it was the opposite for other programs. 1st semester great group dynamics, second ok). You really can't predict who your fellow students will be, but you should be able If your nervous about being alone, just remember that everyone's out of their element, and the majority of your group will be their to support you.
For you more experienced travelers, I would say other then classes and two mandatory meetings you're free to do as you please. Obviously, the less you hang around the gringos and the more time you spend with the locals the more emerged you will become into the culture, and it'll probably help out your Spanish quite a bit too.
There are many places to travel to in Costa Rica especially if you love seeing some beautiful nature. Being in Heredia you're only 30 minutes north of the capital city in San José, the center of the country and all the busses to any destination you're looking for. It may be a small country, but you're not in flatland Kansas anymore. The longest bus ride I had was to Puerto Jimenez (Southwest tip of the country) which was about 8 hours. There are other locations you can check out that will be much closer.
What else do y'all want to know about? 500 colones is roughly equal to 1 USD. Don't think that that makes you a millionaire though. Other then the busses, prices for most common goods runs about the same. Local beer (Pilsen or Imperial) is about $2.50 at the bars, which for people from UNLV is the craziest thing in the world, but being from a small town that seemed pretty standard.
Can I drink the water straight from the tap? Heredia most definitely, but you'll want to ask for anywhere you stay.
Will I have WIFI? You should. The program as a whole tells you not to expect it, but I didn't know anyone in the program who didn't have it in their home. They also have it at the University and most hostels/hotels. Understand that it will be slower, you will get over it in a month I promise.
You should be able to buy most everything you need here in Costa Rica. Save more, pack less. I lasted a year with a fully packed suitcase, 75 liter backpack, and normal backpack. (That's 1 checked luggage (under 50lbs), 1 overhead, and 1 carry on for no extra fee!) Still I feel like I packed too much. Remember you are going to want to buy things when you get here.
SAFTEY- You're going to be fine. I had two situations where I was almost robbed. One was when I was hanging with a Tica in the park at 2 am, and the other was when I was walking alone along a dim lit little traffic road in Puerto Viejo. Common sense tells you, that's my fault. Most of the stuff that happens is petty theft, and usually it is because the individual was not being too bright. Be aware of your surroundings, stick with friends, keep an eye on your valuables and you'll be absolutely fine.
I really hope you didn't read all that, I really did ramble. Just remember that one of the best ways to learn is by doing. If you want to be in the heart of this nation, with easy access to the rest of it, come to Heredia!