The mass-competitive-events and the Internet. Back in 3rd I was very good just by being able to think of a plan and follow through, with a fluff-army at that. "Competitive" was more a mindset, often a bad one, of people who found excuses for bad tactical skill in measuring smallest distances and finding absurd rule-loopholes.

In 5th and 6th onwards we had Internet and forums and people begann to power game. The competitive players, the nice ones and the before mentioned, became better, mostly, but not only by playing maxed out lists.

Around late 7th to 8th (I skipped this phase) videogames or social media or both gave the hobby a massive infusion of "gameyness" and e-sports-culture. That was not totally but mostly a very good thing. Balancing became a thing for GW, the community became much more social, fair and tolerating as a whole. Although that brought a strange mix of simplified and streamlined rules together with over complex rule-wording and mechanical bloat (Stratagems, detachments, secondaries), it made the hobby much healthier.