There is this quote attributed to Nietzsche saying 'Plato was a bore', though I do not seem to be able to find the source for this at this moment. Assuming he did say this, why was this?
A few posibilites I could come up with from likely to less-likely:
One, is this due to the fact that plato believed in 'another world' apart from our world? He believes in ideals and something 'true' whilst Nietzsche opposed this? I think it could be due to Nietzsche advocating the uncertainty, where Plato thought there was something absolute, this is disregarded now by Nietzsche (or we could not reach it if there was).
Or secondly, perhaps it follows from Nietzsche's "The Birth of Tragedy", causing him to compare Plato with the greek figures from apollonian or dionysian characters, and drawing the conclusion that Plato did not have an as interesting life as either those characters or as the writers. [This seems a bit far-fetched, it would not be fair to compare Plato with characters from stories]
And thirdly, Nietzsche wanted to be a modern kind of Plato, a man of similar importance to philosophy but wanted to make it clear that he is NOT plato, and that their views are incompatible.