Well, Jar Jar would not have existed in '77; though I suppose he could have been a guy in a rubber suit. A lot of the visuals from TPM would have been impossible or highly impractical in '77, such as showing all the air traffic on Coruscant, and the huge droid army. Even a single battle droid of that style would've had to have been stop motion in order to be shown walking, like the metal Terminator skeleton at the end of Terminator (1984).

So visually it would have been a lot different. I think its level of success would have depended on how they presented the fight with Darth Maul. If it was a brief affair done in the amateurish '77 Obi-Wan vs. Darth Vader style, then I think the movie would have been doomed, because the story didn't have the "perfect storm" of elements that made Star Wars so great. If they'd hired a good fight/fencing choreographer, and made it look the same as it ended up looking in '99, then I think the movie would have been quite a success (it would have been expensive to rotoscope each frame of such an extensive light saber battle though, and they were faced with a limited budget and time constraints in '77), but still nowhere near the "this changes everything" success of Star Wars; due to the ho-hum story elements.