IE 4.0 was a revelation and IE 5.0 was fantastic as well - effectively it was death knell for Netscape and all competition (earned MS a monopoly suit, but in truth it was a vastly superior browser to anything out there).
They won the browser war. Then, check out the time difference between IE6 and IE7. 5 years. That's where innovation goes absent credible competition.
The web went through a painful stale period from about 2000 to 2005, during which time IE asserted near total dominance. However, sites are just now starting to phase out IE6 support. That means that the technology that has advanced the web over the past five or so years was there all along.
Interest in the web started to return around the same time Firefox started to gain in fanfare, but why? Was Firefox to thank? Those "Web 2.0" apps could have just as easily been developed in IE6 in 2001. It wasn't the technology that Firefox brought that made the difference.
All this proves is that the IE6 monoculture had such an effect that it took a long time for the effects to disappear. But yes IE4 and particularly IE5.x introduced a lot of new stuff that did not become well known until years later, like the infamous XMLHTTPRequest.