As Eliezer makes the point that real utopias will be scary - certainly more scary that my latest attempt. Mainly they will be scary because they'll be different, and humans don't like different, and it's vital that the authors realise this if they want to create a realistic scenario. It's necessary to craft a world where we would be out of place.
But it's important to remember that utopias will not be scary for the people living there - the aspects that we find scary at the beginning of the 21st century are not what the locals will be afraid of (put your hand up if you are currently terrified that the majority of women can vote in modern democracies). Scary is in the observer, not the territory.
This is a special challenge when writing a fictional utopia. Dystopias and flawed utopias are much easier to write than utopias; when you can drop an anvil on your protagonist whenever you feel like it, then the tension and interest are much easier to sustain. And the scary parts of utopia are a cheap and easy way of dropping anvils: the reader thrills to this frightening and interesting concept, start objecting/agreeing/thinking about and with it. But it's all ok, you think, it's not dystopia, it's just a scary utopia; you can get your thrills without going astray.
But all that detracts from your real mission, which is to write a utopia that is genuinely good for the people in it, and would be genuinely interesting to read about even if it weren't scary. I found this particularly hard, and I'd recommend that those who write utopias do a first draft or summary without any scary bits in it - if this doesn't feel interesting on its own, then you've failed.
Then when you do add the scary bits, make sure they don't suck all the energy out of your story, and make sure you emphasise that the protagonists find these aspects commonplace rather than frightening. There is a length issue - if your story is long, you can afford to put more scary bits in, and even make the reader start seeing them just as the locals do, without the main point being swallowed up. If your story's short, however, I'd cut down on the scary radically: if "rape is legal" and you only have a few pages, then that's what most people are going to remember about your story. The scariness is a flavouring, not the main dish.