Two dates spring to mind here.
1987, the year the Raleigh Mustang launched and mountain biking really got into the consciousness of the British public. They were everywhere. It's a pity the bike couldn't go up mountains. The Raleigh Bomber came before and that was more akin to the old SF clunkers, but the Mustang was a step ahead, having 15 hugely wide ranging gears and 'tighter' geometry meant that it was actually pretty good fun trying to find steep trails to ride up.
1991, the year front suspension became mainstream - if you could afford it. (although 1992 was when it really took off). Suddenly you could go much faster for longer, and tackle obstacles and descents fearlessly. And I think that's when a lot of the innovation really kicked off as well.