When water transitions from liquid to solid, typically crystals grow slowly. The angular shapes of the water crystals puncture cell membranes, doing tremendous damage. If it wasn't so, why would frostbite and freezing of your tissue be so destructive? Do you ever notice that when you thaw frozen meat, you often have a lot of extra liquid in the bag? That's what leaks out as the cell walls are destroyed by freezing.

Flash freezing (taking the item down to -30 F or more in very short order) is less destructive. Someone who's even more of an egghead than me will have to explain why the rapid phase change creates different crytaline structures, but it's a well documented and understood event. If you freeze something very quickly and very cold, you maintain most of the quality. Seafood caught, processed and frozen at sea is a good example of this. You rarely get the same result at home.

I do consider freezing to be a great tool for preserving food, but as a chef I have to point out the difference in quality. Of taste, at least, probably not nutrition.