Thanks, everyone!

It's a different spice note, but I find sriracha is always a good addition to Thai curry.

if you blanch your vegetables and chicken beforehand and put in the curry last minute your curry will be less blend because all the ingredients still have there naturel taste. I always finish with tasting and then adding dried chiliflakes, fishsause and sugar till it tastes right.

Lime comes to my mind, and maybe the Thai basil or cilantro?

Use a good chile paste like Mae Ploy brand.

Here's what I do: saute garlic, minced fresh ginger, and onion in toasted sesame oil and canola oil until onions are soft. Add 2 tbsp. red curry paste for a medium curry stirring constantly for about a minute. I then add in tamari or soy sauce, mirin, and chili garlic sauce. You can then follow up with your coconut milk, chicken, veggies, etc. The basil doesn't add any kick, just a light flavor. I have found that letting a curry simmer for 30 minutes brings out its flavor. You may need to add a dash or two of sugar to mellow the flavors.

I agree with healthierkitchen - the spice comes from the curry paste. If adding more does not work, you can make your own with your own selection of chiles. That should kick up the spice. I also think that you need enough salt (fish sauce in this case) and acid (squeeze of lime and/or kaffir lime leaves) to balance the flavor of the curry.

while thai basil and/or lemongrass might add another flavor the kick should come from the curry paste. You might want to experiment with adding more of that to the mix, a little at a time.