As a nod to his comic counterpart, Slade's mask only had one eye-hole, though his "true" face was never shown and his mask was only taken off half-way during his first battle with the Titans; but was silhouetted by a shadow (he was shown to have brightly colored slicked back hair), and then he was seen with his face as a skull with a scar running through his right eye socket when he accompanied Robin against a battle with Raven's father, Trigon.
In the episode "Forces of Nature," however, Slade wore a disguise that closely resembled his face from the comics. He was also changed from a mercenary and an assassin to an enigmatic criminal mastermind (though in one episode he stated that he enjoyed working for someone else), dispatched slews of supervillains and android ninjas to kill the Teen Titans instead of directly coming after them himself. Most of Slade's plans were strictly scientific in nature, though he did summon a fire elemental in one episode.
Slade was shown to be ambitious, calculating, and incredibly sadistic; however, his underestimating of the bonds that existed between all the Titans and his fury over not being able to sever those bonds often led to his undoing. Slade's main goal was apparently to kill the Titans, conquer the city, and quite possibly the world.
In the first two seasons, Slade sought out an apprentice, by starting with Robin in the first season, whom he coerced into working for him by threatening to kill the other Titans with nanobots, and then Terra, whose desire for control and acceptance that he preyed on. Slade was killed at the end of the second season by Terra, who dropped him into a lava-filled pit, and was, as a result, mostly absent in the third season, with Brother Blood, who acted as the main villain.
In the Season 3 episode "Haunted," Slade appeared as a figment of Robin's imagination, due to a chemical reagent that was released from his mask when Robin touched it, and caused Robin to see hallucinations of Slade which attacked him. Robin's mind made the injuries inflicted upon him real. Robin eventually figured out what happened and overcame the hallucination. However, it's revealed that this gas had been triggered from the outside by an unknown party. Slade returned in the fourth season as an undead servant of Raven's demonic father, Trigon. Slade was empowered with vast pyrokinetic abilities, superhuman strength and durability, flight, regeneration, phasing, electricity generation, teleportation, and other supernatural powers, which made him far stronger than the Teen Titans (except for Raven). Slade entered into the deal in exchange for being brought back to life, but was predictably betrayed. Intent on getting his due, Slade helped Robin find Raven, and located the source of power which restored his mortal form along the way.
Slade aided the Titans in defeating Trigon afterward. Slade escaped capture after the world was saved as both he and Robin noted that the partnership had changed nothing. After the fight with Trigon, Slade himself made no more appearances throughout the show, however a robot duplicate of him did appear in the last episode of the series "Things Change," when Beast Boy was searching for answers on why a girl who appeared to be a newly revived Terra seemed to have lost all memory of her past or superpowers. Slade confronted Beast Boy, denied anything to do with Terra's sudden return, stated that if Terra did not remember her past, it's because that she didn't want to remember and that he should leave her in peace. Infuriated by his speech, Beast Boy attacked Slade, only to find out that it was just another robot duplicate.
In the Teen Titans: Know Your Foes interview on the Season 2 DVD, Slade was said to have enhanced strength and an enhanced healing factor. Slade appeared also to have a degree of super speed, as he was able to move faster than Robin could see during their fight in "Apprentice Part II". Slade was well-trained in most forms of combat, both unarmed and with weapons, to the point that he appeared to be Robin's better, and bested him frequently and fairly easily during their fights, but he never delivered a finishing blow. It eventually took all five of the Titans to defeat Slade, though they did so soundly.
Slade also fought Beast Boy once in person, and though the fight was inconclusive, he could have killed Beast Boy at least twice during it. Slade was a bona-fide genius, with great knowledge in robotics, as well as military, political, and subversive strategy. However, Slade was easy to anger, which Robin eventually took advantage of. Slade seemed to possess some knowledge of ceremonial magic (as seen in the episode Forces of Nature). Slade appeared to have access to extremely advanced technology and various secret hideouts, nearly unlimited resources, and a vast army of robot minions.
However, Slade's most notable quality was that he was a master of manipulation and psychology, which allowed him to get his foes to do almost anything that he wanted through shrewd manipulation. Slade was also the only main Teen Titan villain who was still at large at the end of the series.