There is no main factor. It's a combination of factors. Design is very important, as is content, promotional methodology, sales funnel structure, etc.
Ya i accept this... U have to Connect Your Niche with Affilate Banner.... If ppl like your Niche then u got sale easily...
As a visitor to landing pages, I'm personally looking for two things. a) it's professional looking - doesn't need to be cutting edge graphic design, but something professional and credible b) it's got some good content of what the product/service/offer actually is - eg. not padded out with paragraphs of marketing speak - something that gets to the point. That's just me personally though.
I agree with previous posts I only want to add that people most likely click on text links not banners – it’s from my experience.