"What if I was planing on staying in BKK for 30 days then moving to another country for 30 days and didn't have return ticket? WTH was her problem? She could see that it was one way ticket. What if I didn't have it? She would deny me to board the flight?"

Yes she has the right to ask you about an exit ticket from Thailand within 30 days, I personally was denied check-in with Thai at CDG 3 years ago without showing a proof of exiting Thailand given that I was a tourist.

I had to purchase a cheap ticket on AirAsia to Cambodia (I discarded it) and showed it to the check-in agent to let me on the flight.