As authors we can't control the cover, nor the blurb on the back (although one hopes to have input into them). What we can control is the promise we make to the reader on that first page. So it is absolutely essential that the right promise is on the first page, that if it's a racy thriller, either something racy happens, or we're explicitly promised that it's going to happen pretty soon. Similarly a relationship novel should ideally start with the relationship problem being clearly stated.
I must admit I only formulated this idea a few years ago after I'd done classes workshopping first pages with students. The ones that got the most positive responses were the ones where the author's promise to the reader was clear. When I went back to my first novel, Adultery for Beginners, I saw that, without realising it, I'd made the promise there. The opening paragraph starts with the single word: Drat. Isabel and her husband have just made love, but all Isabel can think about is that she's going to have to change the sheets - and she only changed them yesterday. It summed up their stale relationship.
So when you're writing your opening page, think about what promises you're making to the reader, and know that you're going to be able to keep them.