Vader kinda sucked before the Prequels because of ROTJ too. Marshmallow head was one of the biggest disappointments I had as a kid. After Empire, my mind imagined something way more awesome and twisted under than that mask than some pathetic old guy. Vader really looks like a ***** in ROTJ. He's at the height of his villainy in Empire in my opinion.
Also, greygoose and Khev, last time I checked Alien and Jaws didn't have their creators tinkering around with the original films/stories. Even though I don't consider the prequels "canon", there's no doubt that the perception of Vader has changed over time. Jaws and Alien are forever, and finite. Vader is from Star Wars, and is well, not. Once we were given a peek at who was behind the mask, Vader became a lot less appealing. Parred with the prequels (Darth being nothing more than a title), Lucas' edits on the original films (ROTJ's recent "nooooooo!"), etc. it's hard to see that original 80s bad ass.
I'll have moments of nostalgia where I can see Vader in the light I saw him as a kid, but now a days it's kind of tough. I need the rose tinted glasses to be able to see him as cool and not a pathetic cripple that used to be a slave that cried about his momma and Padme. Even without the prequels, as cool as Vader was, once Luke took off the helmet/mask and we saw the pasty dude with a weak voice and bushy eyebrows, Vader's presence sort of diminished.