Dio's simplicity is the key to his success. He doesn't waste pages rambling about some higher ambition for his evil tendencies. He just wakes up after a 100-year nap and wants to get revenge on the Joestars, and maybe drop a few steamrollers while time's stopped on the way.
And as far as villainy goes, he's hardly conventional. He doesn't brutalize any of his lackeys simply to prove a point, he actually employs the carrot more than the stick in his managerial style of his underlings, and when he finally fights Jotaro and pals, it's actually an even and interesting match, rather than Dio dominating by virtue of the power of evil until Jotaro comes to some sappy revelation about love and friendship and kills him with the Deus Ex Machina PAAAAANCHI.
Also, nobody considers Part 1 Dio, with good reason. Stardust Crusaders Dio is the ONLY Dio, and he's fucking amazing. Part 3 Dio is so awesome we forgive all his cowardly douchebaggery from part 1. (Though you have to admit, he made an AMAZING entrance...first thing he does once he gets off the stagecoach is to kick Jonathan's dog, and you KNOW this kid's bound for evil things.)