One standard criticism of the Basic Law is that it doesn't allow the people to ever vote on federal laws directly. This is a point I fully agree with. As I said in my last post, a simple majority at the polls should be sufficient to amend the constitution. But beyond that, we should also have the possibility of initiative statutes and referenda against acts of the Bundestag. Just let the sovereign be sovereign.
The reason the framers didn't include any instruments of direct democracy was their feeling that democracy had gone too far under the Weimar constitution thus creating the instability that allowed for Hitler's rise to power. This story isn't entirely false, but in so far as it's true it's a story of the parliamentary system being unable to sustain the permanent majority it relies on. Hitler rose through the democratic institutions the frames retained, not through those they abolished.
The other standard argument against direct democracy is that the polloi might use it to bring back the constitutionally abolished death penalty. I find it hard to take that argument serious. As an empirical matter, there is no popular majority for the death penalty. If the worry is about some sudden rage, perhaps because of a spectacular crime, that could easily be avoided by allowing the Bundestag to delay the vote for a few months. And if the people really wanted to bring it back they could do so through the present process, surely someone would be willing to pander for their votes.
So this issue is pretty one-sided to me: direct democracy is the way to go and the real reason we don't do it is that the political class would loose power it shouldn't have in the first place.