The answer to this question is by and largely the following:

YES, it's okay!

There are some exceptions (like underage sex for the titillation of readers), but if you're wondering if you can use First Person Present tense, or multiple viewpoints or if you can set the book in 8th century Venice but have flying cars and guns...

YES, it's okay!

If the writing's good and the characters are interesting and the plot is excellent... you'll be fine. If it's a book the reader doesn't want to put down, you'll be fine.