Jess (xjm on Drupal.org) is a developer, reviewer, mentor, language nerd, and naturalist. She somewhat unintentionally developed her own home-grown content management system starting in 2003, and discovered a few years later that Drupal could save her a whole lot of work.
xjm began making novice contributions to Drupal 8 core in 2011 and subsequently founded the Drupal Core Contribution Mentoring program when she figured out that it really wasn't all that scary. She was one of four initiative team members for the Views in Drupal Core initiative in 2012, and has worked as a full-time open source Drupal contributor in Acquia's Office of the CTO since 2013. xjm was appointed as one of Drupal 8 core's release managers during the Drupal 8 beta.
Most significant D8 contribution
I scaled the work of other contributors who accomplished orders of magnitude more than I ever could have on my own: the other three initiative team members for Views in Drupal Core who basically rearchitected not only Views but a lot of core; the dozens of core developers who resolved hundreds of beta and release blockers over three long years of feature freeze; and mentors in the core mentoring program who continue to reach hundreds of new contributors at every DrupalCon, in the issue queue, and in IRC.
My work to get Drupal 8 into the beta and then out of the beta stands out to me as the area where I personally had the most direct impact. In mid-2013 Drupal 8's technical debt had basically spiraled out of control; there was so much innovation in Drupal 8 but so much of it was also just not done. I worked with the committers and community members to take control of this technical debt, by triaging what the real blockers were, planning and leading critical sprints, and making hard choices about how exactly to restrict development so we could stabilize the codebase. Slowly, steadily, we bent Drupal 8 into shape and made it the exceptional release it is now.
Biggest challenge with D8
Saying "no" is hard. Really, really hard. We frustrated a lot of people with the restrictions and policy overhead for the beta, especially since it lasted an entire year. I had to be the "bad guy" and disallow changes that were valuable and high-quality on their own (not to mention often the work of volunteers). There was a lot of pressure to allow this change or that change, and if I made a bad call one way or the other the fallout was even worse. It was a struggle to walk the line and be fair to everyone. The community really stepped up to the challenge and I honestly don't know when we would have released Drupal 8 without these restrictions, but it was difficult for me personally.
Looking forward to most about D8
I'm excited about the innovations we can safely make every six months by carefully following a semantic minor and patch release schedule. I'm also looking forward to triaging and resolving the outstanding bugs that did not block the release, so that Drupal 8 matures as a reliable foundation for sites and contributed projects (and so that core contributors can see the forest through the trees).