I love how it's far more stable and speedier than before. And more seamlessly responsive between devices. Also echo the sentiments above about the forum overall, latest posts feature, and timezone improvements. I'm spending way more time here as a result. Probably too much, oops.
Another simple thing in the forum is the bolding of some of the subject lines on the Recent Posts list. I'm still unsure if this is due to me watching a thread or participating in a thread (or both), but it's really helpful either way when scanning the list to draw attention to threads I'm interested in, among all the other chatter.
The in-house notifications are extremely handy too, and the filterability of the My Comments area is fab (be even better when I figure out what a LocationLevelTwoWallPost is!).
The stories look waaay better, cover images notwithstanding. Just looks far leaner and more modern with decent line and paragraph spacing, and that concise stats block at the top.
Although this won't be of much relevance to non-mods, the moderation process is also more streamlined and is set to improve again as things iterate. It's one of those areas that wasn't just copied over verbatim but studied and improved.
I'm also impressed how responsive the dev team are to jump on issues and squish them. Fantastic stuff so far. Thank you!