I stumbled into Drupal while working on another open source project, and I was looking for the best software to build and maintain websites. After evaluating many options, I ended up choosing Drupal. Drupal did most of the things I needed it to do, except a few. I worked on developing contrib modules for those few things I needed, and in the process, ended up getting deeper into the Drupal community :) I ended up doing my bachelor thesis on Drupal (how to integrate it with a CDN, from which the CDN module originates that I maintain to this day), as well as my master thesis (on using data mining on collected web performance metrics to pinpoint causes for slow sites/apps). That master thesis got me an internship on Facebook's Site Speed team. Since I didn't want to live in the U.S., I declined their job offer and ended up working remotely from Belgium for Acquia… on Drupal core :)
Most significant D8 contribution
I first worked on the authoring experience: getting CKEditor in Drupal core, making sure it is nicely integrated and easy to customize and simultaneously I worked on in-place editing. Then I worked on Drupal's rendering pipeline and caching systems: to make sure everything provides cacheability metadata so we can avoid doing any rendering when possible, and ensure we invalidate and vary cached parts correctly. Depending on what your interests are, either one could be the most significant contribution. But I did none of that alone, at all times I was collaborating with dozens of people all around the world to make it happen! :) The Drupal community is amazing!
Biggest challenge with D8
Getting it done! Almost every subsystem of Drupal 8 has either been rewritten or significantly refactored. Applying the proper patterns, including using the configuration system and providing correct cacheability metadata in each corner of Drupal, and including the ones that have gotten less love in the Drupal 6/7/8 release cycles has definitely been a lot of work. It took a lot of time, but it will have been worth it, because it means Drupal 8 core has plenty of good examples for Drupal developers to follow, and less need for reinvention/duplication when building Drupal 8 sites and contrib modules (for example: many modules provided their own use case-specific caching APIs; Drupal 8's caching APIs have already proven that they are both generic and flexible enough so that those use case-specific solutions are no longer necessary).
Looking forward to most about D8
Seeing the web become a bit faster, as more Drupal 8 sites appear, which will have faster-loading pages out of the box than other CMSs and frameworks :)