This isn't about the movie being dark, nor the bad guys winning as many point out who are fans of ROTS too. It is a point that ESB took ANH to another level, and made it an adult scifi movie that doesnt' talk down to the viewer. Look at the whole cave sequence, nothing is explained, that is the power of a great movie, when the scene speaks for itself, and you don't have Yoda explaining why Luke failed at the cave to the viewer, you just have Yoda saying, "Remember your failure at the cave...." When you first watch the cave sequence and Vader comes out of nowhere, yet the scene is moving in slow motion, you are like WTF? You are challenged as a movie goer, and that to me is the reason I love movies, as I don't want everything spoonfed to me.
ESB made the least amount of money of the OT movies, and I honestly think Lucas abandoned the adult tone and went back to the cheesy-serial tone to caters to kiddies because he thought he could get a wider audience, and he did as ROTJ took in 52 million more dollars then ESB back in 1983. To prove my point earlier, I hated ESB as a kid, and loved ROTJ back in the 80's, and Lucas knew that kids = action figures = ??????
I often wonder what SW would have been had Lucas kept the tone of ESB into ROTJ, and even if he ever did the PT movies, as they would have catered more towards teenagers/adults that kids could latch onto, not viceversa like ROTJ & the PT movies. ANH is a different animal because it is a standalone movie that doesn't flesh out the characters, has a beginning, middle, and ending, and it is true to its roots so being part of the saga was not what it was suppose to do, so in that respect, the movie works just as well as ESB, as those 2 are still my two favorite movies of all-time.
If you watch the movies 1-6 now, you have 2 solid adult movies in Episode IV & V, and 4 entertaining popcorn movies in I, II, III, and VI, and that is why you have so much strife among the SW community, it is so obvious they are two different tonalities of movies, and when it was just SW, ESB, and ROTJ, many 'put' up with ROTJ cause it was the ending of the OT and that is the way it was, but now the PT just continued the cheesiness of ROTJ, instead of the hard-hitting drama of ESB.