First time I saw The Empire Strikes Back was ona TV broadcast back in the late 80s, it was the first SW movie I watched all the way (when I was about 5 I remember watching ANH in a x-mas eve TV broadcast, but I don't remember watching it to the end, I only remember the first 2 minutes) - and back then I knew who Yoda was as well as most characters, even thought I haven't seen any SW movie yet. I did not know Vader was Luke's father, but it didn't get to me as a surprise revelation, somewhat I thought everyone knew about this except Luke, so I wasn't surprised. Maybe if I had watched ANH before, I would have been a little bit more surprised.

I was more concerned on Darth Vader removing his mask, I remember someone told me, obviously by mistake, that in that movie we would see Darth Vader's face, and he would be all deformed and stuff, and I was kinda scared of it (I was 7 or 8 years old). So there was that scene in which Luke fights that "imaginary" Vader, and the masks "explodes" and there's Luke's face behind it, I thought that was it... lol