1. Use of an at least adequate, standard curry paste mixture, selected to be appropriate for the main ingredients. There is more to Thai life than red/green/yellow/masaman curry paste.
   
2. Use of normal-fat (not light) coconut milk, if appropriate.
3. Customization of the curry gravy as appropriate with lemon grass, galangal, tart juices, sugar, fish sauce, basil or mint leaves, hot peppers, to complement the main ingredients and to cover all five tastes..
4. Meat/Seafood/Chicken and a few very carefully selected vegetables, cooked so that each item is tender enough at the time the curry is to be served. Selection should allow for interesting textures or combinations.
5. Service while at the proper temperature.
   

It wouldn't hurt to use a good cookbook: What is the best Thai cookbook?

Poorer curries:

• lack subtleties in taste
• tend to overemphasize one taste, often a poor choice of taste,
• have overcooked proteins or vegetables, or undercooked ingredients
• use soy sauce
• are gluey or runny, or are overboiled