Spot on about the trash. Go to any village that doesn't have trash pickup, and there are many, and the trash is burned or thrown into the nearest woods, torn apart by the ever present dogs. The food also spot on, as Thailand has always had problems with food poisoning. Looking to the crowded places will ensure a lesser chance of food poisoning, as the locals know. Cooking your own food in a family setting also ensures a less chance of getting sick. Some cooks are just unhygienic. Rooms can be had but for a back packer they might be higher than expected, so you need to go a little outside the towns. People are friendly everywhere if you're spending money, otherwise they're much the same as all other places, in varying degrees of friendly behavior, which again is the same as most everywhere. Going to any tourist area you'll be met with all types, as all types of people are tourists, and tourist places are going to be super crowded, of course. Getting off the beaten path to islands out of the mainstream and you'll have more solitude, which is what a back packer should be doing anyway, along with seeing the crowded popular sites. All countries have their share of crapola and good, with more of the good being away from the crowds. See the sights, enjoy the food, then onto the next country.
