... was gesagt werden muss.

Günter Grass has written a bad poem. Andreas Heidtmann's analysis hits all the right points about what makes it a bad poem (in German).

But here's a little linguistic supplement to Heidtmann's remarks:

sage ich, was gesagt werden muss.

The passive voice, it is said, can be used to conceal agency. But here it's not the concealment of agency that is the problem, but the use of the passive to allow one speaker to claim to speak for many.

As with the concealment-of-agency issue, the question is who the active subject of the passive verb "gesagt werden" is. There are two valid interpretations: one is that the speaker (the "ich") is the one who "has to say this." The other is that an indeterminate group of people (in German, you could say "man"; in English, "they" or "people") "has to say this." This latter is probably the salient interpretation for most people, but it also makes clear how Grass makes his complaint seem like something that many people agree with: by not saying "sage ich, was ich sagen muss" but "sage ich, was gesagt werden muss", he lets his claim slip from something personal to something that many people will agree with. 

So the passive here is not a tool of concealment of agency, but a tool for making the claims of an individual sound general, a way to make yourself sound like you speak for a silenced minority that is afraid to speak.

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Afterthought: The salient interpretation of the "was gesagt werden muss" construction is actually a third possibility: the implied active subject is "jemand", as in "somebody has to say it, so I will."

There are two further implications: nobody else was willing to take on the risk of saying it, and "I am the one who is courageous enough to sacrifice himself by saying it."

This is connected to the overall tenor of the poem, which focuses less on the critique of Israel than on Grass's perception of himself as a victim of the criticism that he expects to receive for having written the poem.

In this respect, all criticism of the poem for its representation of Israel fulfills one of the poem's goals (I would say "implicit" goals, but in fact, it's pretty explicit): that Grass will be criticized for it. One could call this masochism, but what it really is a rhetorical use of the passive voice to stylize oneself as a victim of the criticism one expects to attract.

The rhetoric is quite powerful, in fact, as it defuses all criticism of the poem's contents by anticipating it and putting those who do criticize the contents in the position of protecting taboos that only Grass has had the guts to break: somebody has to say it, and I will say it, and if you criticize me for it, then you are part of the reason that nobody had said it before.