Silence. Just silence yourself. Zabuza a joke? No. Zabuza is an elite Jounin who's specialty was assassination. Not giant ninja battles like what we've seen from nearly all of part 2.
It was said Zabuza could kill you before you even realized it. Pretty God Damn impressive if you ask me. Also, age Nine-ish he killed the graduating class a year ahead of him. Every. Last. One. Yeah, he's nothing. (Sarcasm).
His physical strength is damn great too! He easily tosses around Kubikiribōchō and weilded it with a single hand, meanwhile Suigetsu struggled with the sword.
His mist technique and mastery of silent killing was so great that he was nominated by Kakashi to be the biggest threat in the war. Over the rest of the Swordsmen that people find to be "Superior" simply because their blades have some ability.
The Demon of the Mist was a powerful shinobi, how powerful? Mei mentioned that Sasuke defeated him and then acknowledged Sasuke as a threat.
Just because a ninja lacks "flashy" and "powerful" jutsu doesn't make them weak. While he certainly isn't the strongest, Zabuza's attack style is a deadly one indeed. And again, he's not meant to be taken as a shinobi who tanks and attacks directly. He's an assassin.
And don't forget as the first villain, he couldn't be as outlandish as the rest. He had to have more ninja like ties and skills. Rather than having a puppet body, five hearts, or magic eyes that seem to do whatever they want.