Frozen fish? Feh, if it wasn’t swimming this morning I ain’t eating it. 
And powdered wasabi, powdered wasabi: double, nay, triple feh!
Actually, most Japanese don’t care. [sub]Well, you know, you nitpick where you can…[/sub]
That being said, there’s not much left to say after Zenster has stormed the place. So, I will reiterate a point that I think many people take for granted and is very, very, very important: slicing.
Not only should you not use anything short of razor-sharp, you need to have the proper technique. Too many people make a “pushing down” motion when slicing with a knife. Sashimi knives are very long and that’s in order to facilitate a horizontal motion. You should exert a minimum of downward pressure on your fish. This will not only make your fish look nicer, it will also taste better, as Zenster mentioned.
Oh, and don’t wash the fish in water, it’ll lose a lot of flavour.
Second, as much of a food snob as I am, I think going with stock dashi is best for the beginner. Basic dashi only has three ingredients: water, bonito flakes, and konbu seaweed; however, making a good dashi is much, much harder than it seems. Plus, bonito flakes cost a fortune in Canada. Well, eastern Canada.
