When you're building supertalls, there are other problems to worry about than just making sure they don't fall down. One of the biggest is how to get people up to the top in a reasonable amount of time. If you've got a slow lift the 125th floor might as well not exist.
This graphic, put together by the Financial Times, shows how some of the world's tallest towers' lifts compare, and it's not as simple as "taller = faster."
The Burj Kahalifa, tallest tower in the world, has one of the slowest lifts (a mere 10m/s) compared to other towers that are in its general ballpark. And the CTF Financial Centre skyscraper—due to be completed in 2016—will have the fastest at a blazing 20 m/s. These impressive people movers aren't just something you can whip out of nowhere though, and to make sure they can handle the incredible heights and reasonable speeds, they have to undergo lots of testing.
Building up ever closer to the stratosphere comes with all kinds of challenges, and making it possible to really use those upper floors is maybe one of the least glamorous, but most interesting. You probably won't see the CTF Financial Center getting as much name recognition as the Burj Kahalifa, but I know which penthouse I'd actually have the time to visit. [The Financial Times via Quartz]