How to pick mountain bike pedals (platform)?
We can't recommend which model to buy, but some good things to keep in mind are the following.


Platform size: Width and thickness, if you have wide feet you may
want a slighter wider pedal. And being overly thick means you will
probably catch them on rocks more often.
Pins: Removable metal pins or molded in pins, i prefer removable metal ones, which are usually found on slightly more expensive pedals. However when you smack your shin on a metal pin it does quit a bit more damage than a plastic one and they can also scratch your legs up when standing over your bike. Some metal pins are also hollow, improving both grip and leg shredding ability. Some are very sharp and some are well rounded.  
Bearings: Sealed or loose ball, cartidge or DU Sleeve. Sealed is generally better as it will keep dirt and water out making sure your pedals are buttery smooth. A lot of people are unfamiliar with DU sleeves but they are actually in a lot of different pedals, sometimes under a different name. Basically they are a Teflon impregnated tube that is in contact with another tube of the same material, the have a longer break in period but both self lubricate and are virtually maintenance free. Many pedal manufacturers use both a sealed cartridge bearing and a DU sleeve.
Concave or Flat: This is more of a personal preference, but a concave (curved inward) surface is suppose to help keep your feet planted and increase traction on the pedal.
Material: Metal or composite, generally metal will last longer, and the pins will not be worn down or sheered off on rocks if they scrape. But an expensive composite pedal could certainly outlast a very cheap metal one. There are also composite pedals with metal pins, offering low weight and good grip. 
Price: Kinda obvious, i will say this though, there are pedals out there that are every bit as good as other 3 times the price. Just because they cost $100 and have a name brand doesn't necessarily mean they have any more features or durability. 


These are what i ride and i have zero complaints. Love em, super thin, and their affordable should i ever break one. Best thing this company makes in my humble opinion. A good example of a metal pinned pedal



And here is an example of a plastic or composite pedal with molded in pins.


You can easily see the difference in thickness between these two. The concave isn't as obvious due to the angles but you can see how the curve downward toward the center of the pedal creating a bit of a dish effect. While the Origin8 pedals in red are rather flat in comparison.

Lastly, what type of riding your doing will also have somewhat of an impact, generally dirt jumping riders prefer a stiffer axle so that the pedal doesn't spin when they are air born with their feet off the pedals. General MTB riding this is not the case and not as important.