Some people didn't know who Yoda was (thinking that old green guy was Yoda's servant or something), and some did catch up quickly and figure it out it was him. While it was no longer a surprise when the movie was released, since there was a lot of publications and pictures of Yoda, some people didn't see it, and Yoda is not featured in the original trailers. But yes, some were fooled. ESB was the first one I've seen, and for some reason I knew who Yoda was, I just don't remember where I've seen it or who told me.

Btw, I didn't know Vader was Luke's father, and when he told him, it didn't struck me as a surprise, because somehow I felt that the audience already knew it, that everyone knew it but Luke, so I was like "oh ok". What really surprised me about ESB, and made me quite pissed back then, was that the movie had no real ending. I did feel disapointed, it was the first time I've seen a movie with a hook to a following movie (I belive I'd watch Back to the Future 2, the first movie I clearly and fully remember seeing at the theaters, a few months later - and even that one had some scenes from the next one).