Opening Scene Checklist

The opening scene of any book is the most important. It's the chapter you'll most likely spend more time working on than any other.

As important as the rest of the story is, and as much as you need to write a story that grips the reader to the very end, providing sufficient conflict and pace to keep the reader turning the pages, with an ending satisfying enough to make the reader want to read your next book, if you haven’t managed to hook the editor / agent / reader in those first few pages, all the rest will be wasted effort.

Your opening needs to accomplish several things:

Introduce the main characters. This is perhaps the most important aspect of all, as readers need to care, and the only way they will care is if they understand who this story is about, and if they identify with your central character(s).
What’s at stake? Introduce the central conflict and theme of the novel. Give the reader at least a hint of what this book will be about.
Set the scene. Where in the world is your story located? Give your reader just enough description and world building to picture the background, but don’t dwell on it. This is the moment to capture the reader with action and dialogue, not lax wyrical.
Start the scene at a point of change. This is not the moment to have your hero or heroine in introspective mode, re-living the past, or waking up, getting dressed, brushing their teeth, making a cup of tea. Start with action, in the present, at the moment he or she starts on the path of this story. Not five years before!
Polish, polish, polish! More than any other chapter, your first needs to be free of typos, poor punctuation, unclear POV shifts, incorrect word usage.

What not to do:

Info dump. Do not bore the reader to tears with your character’s entire back story / life history. That is for you to know and the reader to find out - slowly, and in bite-size chunks.
Climactic action. Do not start at such a high point that you cannot top it again for the rest of the novel. You’ll only leave your reader dissatisfied. Leave the climax for where it's supposed to fall in the book.
Cheat the reader. Similarly, don't start in a dramatic style purely to capture the reader's interest, unless you intend to carry that style throughout the novel. This goes especially for the dream sequence opening - a highly dramatic opening, which pulls the reader in - and then turns out to be nothing but hot air. If you're old enough to remember Bobby Ewing in the shower, you'll know exactly what I mean.
Avoid stereo-typical beginnings. Editors, agents (and even readers) have seen so many of these they'll most likely yawn is you do the same: the alarm clock waking your main character, the car accident between hero and heroine... Aim for a unique and memorable opening.