Meenakshi Ammal in her books Cook and See, talks about Sambar tastes and textures, which she says are personal preference.
These posts might also be helpful:
Sour, Salty, Hot?
Some prefer their sambar a little sour, some a little hot and some more salty. Sometimes, some varieties of tamarind are more sour than others, some chillies are hotter than other chillies. Experience, personal taste and discretion should determine the amount, the number and the quality.
Green chillies are not compulsory and may be substituted by red ones.
Thickness and Thickening
If red gram (toor dal) is used freely, there is no necessity for thickening with rice flour as the sambar would be thick by itself. But if the quantity of dal is not enough, and if the tamarind is old and dark in colour then you can use bengal gram flour (besan, chickpea flour) for thickening the sambar, to give it a good colour.
If masalas are liked, when you are making the sambar, fry 1/2 tspn poppy seeds, 1/2 tspn anise, 1/2 finger length of cinnamon stick, 4 cloves and 4 cardamons, and use for flavouring the sambar. Add the powder with the vegetables as they cook.