In my opinion there are two major weaknesses in the Weimar Constitution - firstly, the context for its creation and, secondly, the P.R. voting system...
The Weimar constitution was created by Wilhelm the Second. He created the constitution and the democratic government as an alibi; he saw the impeding defeat and decided on trying to create a scapegoat that could take the blame for the war and everything that went with it. For the past several years he had tried, time and time again, to persuade the German public to put up with more and more day-to-day hardships always saying that it was for the all for the good of the war effort and that Germany was easily on course to winning the war. This created a lot of resentment among the German people and when, after all their hard work and labour, they ended up having their style of government changed, being put under huge blockades where very little food or water was available and finally, less than a year later, being 'backstabbed.'
The Weimar Constitution in booklet form.
Weimar Constitution from 1919
Reichstag Polski: ReichstagThis resentment had to be directed at someone, they needed to blame somebody and the logical choice would be the new government which has no experience at all who, within a year of their inception of the government, has already signed over your land, made you officially Polish in origin and backstabbed you in a war where you haven't lost a single battle on your own soil.
The new government, as it was intended to appease the allies, had to be something that the Allies could sympathise with, not a dictatorship. Therefore, Wilhelm II decided to create a Western-style democracy which no-one had ever seen in Germany before and was an 'Alien' system. It was seen by the majority of people as a bad move...