Complete Monster: Ultron is a rogue A.I. obsessed with nothing less than the total extermination of not just the Avengers, but all of humanity. After building the android Vision, Ultron first has him attack the envoy of the President of the United States, nearly killing both the President and his agents, then organizes the attempted mass shooting dozens of innocent people, children included. In another instance, after Vision has gained sentience and betrayed his evil ways, Ultron holds the comatose Wonder Man hostage to try to force the Avengers to destroy Vision's capacity for emotion and turn him back into his slave, after which Ultron tries to kill them all. In his crowing moment of villainy, after failing to bombard the Earth with nuclear missiles, Ultron attempts to spread a technovirus across the planet, then watch as every machine on Earth comes alive and wipes out every human they see, paving the way for Ultron's mechanical tyranny to begin.
Also criticized was the fact that the Big 3 of the lineup were absent (again, due to legal snafus regarding planned films; in Cap's case, he was also supposed to have gotten his own series just before this, but Marvel's bankruptcy put the kibosh on that); and yet, they could have followed up the brief Avengers cameos in Fantastic Four, or have the Hulk from his UPN series appear, etc. etc. Instead, they turned it into a mishmash of Batman Beyond (what with the vaguely futuristic setting) and Power Rangers (with the whole armor dealies that half the team wore). Again, Marvel's bankruptcy might've had something to do with this (the armor was likely for then-Marvel subsidiary Toy Biz's line of figures for the show), but still, this show was just a trainwreck.