Of course it was the quality of the search results thanks to the algorithm (Page Rank) that at the time was unmatched and amazingly resilient, compared to the competition, against the primitive SEO tactics of the day (key word spamming etc.).

However, the lean interface without blinkentags and ads was definitely a selling point. Also, IIRC, the guarantee that you'd only get sites that actually contained all the words in your search query (that feature is long gone, too, of course).

I guess it depends how you define "selling point" exactly.

The interface and speed were great, no doubt. Did you ever encounter another search engine that produced similar or better results that you otherwise would have used, but Google's interface sold you? I never did, so it wasn't a selling point for me.

There was a time when google's search web page was under 16kb.