Star Trek: Voyager did a great job of establishing some major villians during its seven-year run. Species 8472 and the Hirogens come to mind, but when you think about really scary enemies who could give Voyager a run for its money, the Kazon don't even make the top ten list.
Introduced in Voyager's pilot episode, "Caretaker," the Kazon were meant to be a formidable species with an aggressive, territorial nature. Though they tried to come across as frightening (and even managed to give Voyager some serious trouble in a few episodes), overall, they just weren't that intimidating. Sure, they brought the weapons to the battles, but, in the end, they just weren't compelling enough to be the ultimate warriors to defeat Voyager or really even stay in the battle very long. They were phased out as the series went on, though, they appeared in every season of Voyager.
Perhaps it was the Kazon's desire to be thought of as formidable that led Vulture to call them "the Gamma Quadrant's dollar-store version of the Klingons). Though The Kazon brought some firepower to their battles, they were more into machinations that resembled Wile E. Coyote's attempts to capture The Roadrunner, such as when they used Sestra in a foiled attempt to overthrow and capture Voyager. The ending was something you could see coming from a mile away.
When compared to Klingons, if that's even a possibility, the Kazon tried too hard to be villains whereas the Klingons' appearance, fierce fighting abilities, and natural warrior nature truly made them a threat. Though the Klingons have resolved their differences with the Federation, the Kazon just faded into the background and are barely considered a blip on the screen in Star Trek history. It's doubtful we'll ever see these dollar-store Klingons as terrifying foes if they do make an appearance in Star Trek again.