As an avid user of mapquest back in the day, the user experience that google provided (tiled maps that could be panned/zoomed seamlessly) was absolutely revolutionary and the clincher for me.
Mapquest back then had the clunky “click the big arrow” to move the map and reload the page navigation...
The instant utility was essential to its massive popularity, it basically ushered in the new era for real-time Consumer mapping. Everything preceding it was clunky, slow and "turn based". I can't recall what the state of MapQuest was at the time, but it's unlikely it had the real-time UX or utility of Google Maps.
Was a real eye-opener of "so that's how you make Maps fast" at the time, just pre-render the entire world at multiple static zoom levels. Which wasn't an option (disk cost) for anyone developing their own in-house mapping Services at the time. For my next GEO project I did the next best thing and used their Google Maps API to change it to make map tile requests to our ArcGIS App which dynamically cached map tiles on the fly, so whilst it didn't pre-render all tiles, it provided a nice "Google Maps"-like UX for popular areas of our region.
Multimap and Streetmap in the UK both had prerendered tiles at multiple zoom levels years before Google. I'd developed a smooth "slippy map" at my then employer (waterscape.com) before Google Maps was released, using Flash and the Ming authoring interface, but IIRC Google released before we did.
