Yes there were checks and balances. The Weimar Republic was a Presidential system, very loosely resembling the American one. In a way, the Weimar Republic was more democratic than modern Germany. For instance, there is hardly any Separation of powers left in the modern German political system as the executive and legislative are basically the same.

What really turned Germany into a dictatorship was the Enabling Act of 1933. The act itself violated the Weimar Constitution but that was deemed acceptable by contemporary political theorists as the act was pushed through with a two thirds majority, which would have been sufficient to change the constitution anyhow. Anyway, this act gave tremendous powers to Hitler, it would allow him to basically introduce any law he liked even if it violated the German constitution and prevented any checks of his actions.

The fundamental difference between the political system before the war and now is that the Weimar Constitution was solely based on Legal Positivism. According to this view, governments can create whatever laws they like and also abolish any rights citizens may have previously had. A law is simply something man has created and he can also change it whenever and in whatever way he wants. In 1930 there were many German extremists parties present in the Reichstag that were all too wiling to abolish the democratic system.

The modern German constitution has incorporated many aspects of Natural law. Humans simply have certain inviolable rights. Those parts of the German constitution may never be changed. This is probably the one fundamental change in the political system. Moreover, the horrors of the war made the German population more reluctant to vote for extremists parties again. Anti-Semitism and other forms of racism were far from dead after the war, but the economic prosperity and need to keep an Alliance with the Western Powers to keep the still hated Soviets at bay during the Cold War simply made a democarcy more appealing for the Germans.