Shoes are another one of those person preference things; personally I think Shimano shoes are the best because they fit my feet the best. In general though:
>Should have a stiff sole for efficient pedalling and to provide a platform for your foot.
>I prefer velcro straps/buckles over laces, as in my experience laces can easily get caught up in the drivetrain.
>Waterproof/resistant materials if you ride wet conditions.
>Grippy studs like football boots so you can still climb up the stuff you can't quite ride up.
>Check that the shoe is compatible with the cleat & pedal you plan to use (most should be but still worth checking)
>I don't think shoe weight is critical to a trail rider, comfort is more important, but the high end stuff will have carbon soles etc.