As a perspective reader, the trailer would have to be of a high quality for me to take notice. It would have to offer something beyond what I can get from reading the blurb and viewing the sample art elsewhere. Since it's visual, it will be judged primarily on its looks, so that's why its important it looks professional.
WriteKnight's making of video is brilliant, I love that sort of stuff... however, I found his 3 and a half minute long trailer to be rather chaotic, long and directionless.
The good thing about trailers is there's no rules. People tend to advocate a safe and reliable "formula" (just like with log lines, queries, etc), but in the end it just comes down to one thing... getting the viewers attention.