Quality of frozen salmon (fish in general)
Some species of salmon are better or worse at freezing.  Pacific Sockeye freezes very well and is your best bet if you're buying frozen.  It's also the most expensive.  Pink salmon doesn't freeze well at all (but is delicious if you can get it fresh!).  Others like Chum are somewhere in the middle.

Most "cheap" salmon that you get in North American supermarkets is farmed Atlantic salmon.  It holds up well to freezing, but is generally pretty gray and bland to begin with.  Avoid it.