The thread title spoiled the surprise of seeing Thanos show up, how can anyone argue against that simple fact?
A spoiler is a plot significant element intentionally hidden until the movie is viewed. Thanos' presence as a character had been publicly announced and casted by Marvel. Gaffers can't be held responsible for what other members know about in a film. If someone knew nothing about GotG and planned on seeing it tomorrow, but I told them one of the main characters was a raccoon, should that constitute as a spoiler?