What made Boba Fett so popular was the mystery surrounding him. When he made his first appearance, it was as a special send-away toy after ANH came out and before ESB was even close to ready to go. (No, I'm not including his appearance in the Holiday Special). All of a sudden, there were stickers on the carded figures showing this guy in this grey/blue armor outfit with a pretty damn cool helmet and weapons and swooping downward on a jetpack. No one knew who he was or what side he was on. But they wanted the figure as it was part of the SW canon. Then we see him for short bits in ESB as he is the one person that was able to track down Han and take possession of his carbon frozen body. I still think it was Boba that tracked Han down and called in the Empire although some people argue that the Empire tracked him down. BS.

His voice is also very cool (pre 2004 DVDs) and very underused leaving you wanting to know more about him. Then, after the OT was finished, all this EU stuff came out alluding to previous encounters between Han and Fett and even postulating that they are somehow related. There was all kinds of speculation.