Slurping from a cup of tea, so the sound is heard. Better yet, pouring the tea into a saucer and then slurping the tea. Easily noticeable in Irani cafes in Hyderabad.
Eating rasam and rice off a banana leaf-plate using your hand to scoop the mixture, including the liquid. It’s an art form. Curd and rice is another combination, but not as difficult. [1}
Ordering by-two anything, a concept which defies the laws of mathematics. Ask for a soup by-two and they’ll give you portions (in most places) that put together will be greater than a single order of soup. Same with tea or coffee. I also love the 2/3 concept, which is two orders of soup (for example) divided by three people.
Is this stuff unique to Indians or what?
 — In general, eating food with your fingers is another speciality that I think we’ve mastered. The Chinese have their chopsticks, the Westerners have their forks and spoons, we use our fingers.