The artist list displays all artists that appear on at least one non-compilation album. Is there a preference somewhere to change that to display all artists, i.e. including those that only appear on a compilation?
You can't do that in Artists view. The only way to do that is to show the Column Browser (View > Column Browser > Show Column Browser, or Command-B). Unfortunately, in iTunes 11, the column browser is only available in Songs view, or when you're viewing a playlist. But it will show all the artists, even those in compilations, as long as you choose View > Column Browser and uncheck Group Compilations.
iTunes 10 was much better for using the Column Browser, alas.