Fantasy SciFi Opening Chapters?

Beginning with action is still the best way. If you don't grab the reader's attention quickly, you'll never keep it for later. That doesn't mean that you should be blindly throwing out many names and details to make it confusing and I don't remember any books right off hand that do that. This opening scene should be hard-hitting and low on details; those can come later. Most readers will not sit through a massive info-dump before the story actually begins.

I think the perfect opening would be either deliberately mysterious (so we want to find out about these people, even if we don't know anything yet), or full of tension, like a chase scene (which grabs you from the beginning AND gives you an opportunity to get a feel for the characters)

I find that if there are lots of explanations at the very beginning, I get a little bit bored. I like having an action scene to get me interested, and to make me desperate to continue reading.

But neither opinion is wrong, everybody has their own preferences

This is what I've learned from a lot of research and I'll tell you what you're confusing, or misunderstanding, however you want to put it. ;)

You need to grab the readers attention, especially if you're writing a Young Adult novel. Teens have a short attention span, so young adult books need to grab their attention from the beginning. Adults are more likely to give it time to play out, but you still need something to keep them reading. This isn't an opinion, this is a fact from studies that have been done.

Now, don't confuse needing to grab someone's attention with starting with action. Action doesn't mean what you think it means. It's means that something needs to be happening. Action doesn't mean a car chase or a sword fight. That something could mean a girl who lost a very important ring and is worried about what her mother will do when she finds out. In that kind of scene, it will be the tension that keeps you reading. Starting with action simply means that something needs to be happening. Your character can't be sleeping and waking up to have breakfast, for example. Even literary fiction has something to keep you interested at the very beginning.

So, yes, something needs to be happening to your characters in the beginning if you want to keep the average reader reading. Think of "action" in terms of conflict. You need some kind of conflict. Conflict doesn't have to mean two people arguing or fighting. It means there is some kind of friction.

But, lets stick with what you think action is: now, lets broaden it. Your novel needs to start with action, tension, or conflict. Something needs to be happening. Don't think conflict means two people have to be fighting or arguing. Conflict can be external or internal. Tension is another word for suspense. Either way, something needs to be happening. You don't have much time to make something happen: you have about 5-10 pages to make something happen if you want to keep your reader reading. Character development comes throughout the novel, not just the beginning. So does plot development.

Reread some of your favorite novels and try to pin point what the author is doing that's making you want to keep reading. The one I'm currently reading starts with action in the sense of what you're talking about. A book like the first three harry potter novels start with conflict: they show harry at home and show his relationship with his guardians and how he doesn't' want to be there. That's conflict.

Either way, you can build up to it, which is why you're not expected to make something happen on the first page, but you need to get to it quickly because most readers won't sit around waiting for something to happen.

And, you're right about jumping into the middle of a sword fighting scene or car chase. If the reader doesn't know who the character is then they probably won't care, but if the action is good then they'll keep reading. That's what you want. You want them to keep reading, because over the course of the book you'll eventually learn a lot about the character. One mistake of a lot of first time novelist is that they add action thinking they need it, but that action has absolutely nothing to do with the plot. That chase scene or sword battle needs to be something that moves the plot forward and not just be there because you supposedly need action. There needs to be a reason behind that scene and everything you write for your novel.