I've been in a few good tributes, and left for various reasons.
For me, it's about one thing - it's not the looks, not the sound, not the stage antics - but it's about the "experience". That's what makes a good tribute. A great night out. A good resemblance to instruments, costumes, sound, looks all help sell that experience. I also fully believe you need to have a bit of yourself in their too to make it appear authentic.