The power of the executive to veto legislation is an important check on legislative power. The spectacle of one elected official countermanding a great body of elected officials may seem unbalanced, but it is a purely negative power, and little harm can come from laws unmade. Nor is there a need for legislative override by a super-majority of the legislature. If a law is sorely needed, and that belief crosses party lines, and a madman holds executive power, the law can be passed by initiative and/or the executive can be recalled.
If the executive branch believes a law is unconstitutional it can challenge its constitutionality in the courts. It must never refuse to enforce a law that sits on the statute books or otherwise frustrate promulgated laws. This is because executive officers are charged with implementing the laws with uncompromising fidelity. They have no discretion to pick and choose which laws they will enforce.