If we're talking about the front page, or browsing through categories, then the icon is definitely the main draw for me. I'm unlikely to go by title, because I'm really not very good at coming up with titles, and I'd like to give other creators the benefit of the doubt in that department.
I think I picked up the majority of the webcomics I follow either from recs, or from seeing their art/pages via sites like Tumblr, though, so overall I wouldn't say the icon is the most important factor, but it still plays a significant role here on Tapastic.
I do think it's really important for the icon to be representative of the art, though. I don't mean black and white comics need a black and white icon, but I've come across a lot of comics where their icon/banner is the very best of their art, strategically cropped to mask the creator's weaknesses. Or a beautifully painted icon for a comic with pages that are basically rough sketches. I understand wanting to use your best art to advertise your comic, but an obvious disparity in quality between the icon and pages is far more off-putting than a more representative icon suggesting a lower level of polish.