This is off the top of my head, and I'll come back and add more when I remember it. I focused far more on Hitler's relationship with the Weimar than the system itself, so I'm afraid I can't offer the greatest insight.
One - The Weimar system itself was not even liked by the democratic parties that were part of it. See if you can find any good quotations for it though, as evidence for dislike at that stage isn't easy to find. You can then explain how it develops into acceptance of the Nazi's extreme idealogy, where you can show the Nationalists' and the establishment's ultimate sympathies with an anti-democratic system.
Two - Simple one, talk about the inherent weakness of proportional representation, manifest in the way minority parties could grow and shrink in size very rapidly (heaps of statistics available on that one). Also the way that the President or the President's office could be manipulated either by those surrounding the President or through legislation (eg. when Hitler merges the Chancellorship with the presidency).
Three - The Treaty of Versailles. This is a good one as there are two distinct sides to the VT argument, this is the side where it caused the Weimar to be too weak.
You can argue that the VT was a compromise treaty; the French thought it wasn't punitive enough, the Americans thought it was too punitive as it was.
The Treaty was punitive enough to cause a dent in national pride, at this point I would simply list those things (War Guilt Clause, losing territory in both West and East, having to pay embarassing repayments).
This gave rise to the Dolchstosslegande (Stab-in-the-back myth), widely used by Nationalists and Nazis alike, and a popular belief that was connected directly to the November Crimina