Same here. I like to be able to tighten around my ankles, while leaving my toes loose. My shoes have 2 Velcro straps, but a ratchet closest to the ankle.

Dynamics of Riding...
 

...are where clipless pedal/shoe systems shine. Remember that mountain biking is a progressive habit. What begins by riding tame crushed gravel trails through the local park - where you could ride on platforms with flip flops, eventually becomes radical jumps, climbs over rock gardens, wicked descents and hairily banked curves. The bike is no longer oriented "up-and-down" with your weight over it (as if you were riding in the church parking lot), but rather, is oriented at all kinds of crazy angles - pointing upward, leaning to extremes, with your weight wherever you can place it to maintain some degree of control.

Now, let's talk about moisture, mud, leaves, debris, sand, and any of it between you and the bike at any time. You need grip and plenty of it, or you will rack yourself badly if you lose contact with the bike - with gouges on you shins and ankle bones, or worse...

This is where clipless pedals make a difference. While positive contact is maintained, you can power through and adapt to whatever happens that is outside of "normal", however rapidly the conditions change. You can power through a stall, you can get serious air to avoid getting the wheels wedged, you can guide the bike up and over obstacles with the confidence that you are attached to it. Smooth transfer of power is uninterrupted.

I guess that confidence is the byproduct. You'll need confidence.

Others will say that platform pedals with the little knobs, and the right selection of shoe, can give very good grip. This is generally what downhill riders use, I'm told. I don't live near ski resorts or in the West.

Like someone else said, just go and do it. You'll figure it out.

My preference is SPD dual sided M545 aluminum caged pedals for mountain riding. I've used the M424's, but quickly smashed the cages. I also use SPD on my utility bike and touring bike. For shoes, I use lace up Shimano MTB shoes with recessed cleat area. I also use Lake sandals for warmer weather, and I like them alot - they have a capped toe for protection, but give plenty of ventilation. They dry out okay, too, but they're pretty heavy.

Much the same as clipless on anything else. Your feet are attached to the pedals, which has advantages and disadvantages that vary based on your skill, terrain etc, and what you want to do with the bike.

The main advantage of clipless is that your feet are attached to the pedals. That means you can generate some power with the upstroke, things like bunny hops should be easier (I don't do them, so can't say from experience), and your feet won't slide off the pedals.

The main disadvantage of clipless is that your feet are attached to the pedals. That means if you start to tip over and forget to unclip you're going down. It also means if your bike starts to fishtail in the mud you can't just put your foot down, you have to unclip first. If you're slowing to the point you start to tip over and forget to unclip, you're going down.

No way to answer that, it's like asking what bike you want.

I always recommend starting with SPDs, even for road biking, then moving to other systems only if you're having specific issues with the SPDs. Because if you can live with SPDs, you'll be rewarded with most flexible range of pedal and shoe choices. I like them just because they seem to develop the fewest quirks over time like creaking, though I'm primarily a road biker so I can't comment on how they fare for hardcore trail riding. Plus for $40 it's hard to beat the M520 as a tryout pedal.