There are many intestinal parasites, bacteria and viruses which can be transmitted via food, water and eating utensils. Usually, the most serious one is hepatitus A.
Hygiene is often not good in Thailand. The ThailandGuru has gone to the toilets of respectable restaurants in foreigner areas and shopping malls and seen the kitchen staff leave the toilet cubicle without washing his hands, and looking inside didn't see any toilet paper or even soap. (Thais often use only water and soap, not toilet paper, to clean themselves.) Vendors of sliced up fruits on the streets often come from slums in Bangkok.
Dirty glasses and eating utensils are something you should check for every time, as well as debris in water and ice.
Shellfish such as mussels, oysters and clams should be avoided. Steaming them is often ineffective in making them safe to eat.
Make sure meat is cooked well, especially pork.