Let's not forget about all the monstrous heroes out there - Swamp Thing, The Hulk, The Thing, Etrigan, Man-Thing, Nightcrawler, Beast, Blade, I Vampire, the Doom Patrol, The Spectre, and scores of others. Even Phantom Stranger has a few traditionally villainous features in his look, notably the pure white eyes.
Black Widow and Catwoman kept fairly villainous looks after their conversion to heroes. A woman in a sleek black catsuit always says "dangerous sexuality," which of course reads to most of the comic book audience as "EVIL IT BURNS."
One of the villainous facets of Drax's earlier look is the high collar - more elaborate capes tend to suggest regal supervillains. The Marvels are an exception here, as is Martian Manhunter.
Odd hairstyles were usually a villain convention back in the 70s and 80s, especially as the punk scene became notable. Every multicultural 80s comic book gang had one guy in a mohawk. Wolverine and OMAC defied the era's weird hair convention.
Speaking of hair, totally bald usually means villain, often a mad scientist type. Professor X semi-defies this, depending on how sociopathic you might feel Professor X is (the answer is "Entirely").