The use of a different radio band, therefore less contention in the existing mobile bands - less congestion results in better speeds overall.
Reducing the range of base stations. shorter range means less clients, less congestion and therefore better speeds, whilst also deploying them more densely to cope with wider areas and higher bandwidth densities. Also, shorter range reduces the power requirements, meaning mobile devices will have longer battery life (nothing magical, probably not even noticeable to the average user), or it can be built into smaller/low power devices such as IoT.
Utimately, 5G is irrelevant to end users until it's actually deployed widely. Just like 3G and 4G, the end user has no impact on the deployment of the network other than the demand for it. So, all the hype around 5G is almost entirely marketing, politics etc. It only really matters once 5G is deployed across the areas you visit daily, and until then the previous generations of mobile connectivity will continue to serve just fine.
Your suggestion about a football stadium is an interesting test case. Ideally, an area that size would be served by up to a dozen base stations, spread throughout the stadium. Compare this with a single 3G base station that would cover the stadium, plus a large portion of the local area, and you can see the pros/cons fairly easily. But how many people are surfing the web whilst watching a game? or taking calls, answering texts etc. Very few during active play time, but there'll be large bursts of traffic in any breaks in play which will stress the older mobile generations to breaking point whereas 5G is designed to deal with that scenario fairly well.