We had been waiting at 0.18.5 for a while because 0.19 came with some federation bugs (and obviously it would be extremely embarrassing if this instance had issues in that department).
The biggest changes for users are a new scaled sorting method that should prioritize posts from smaller communities (no more Risa dominating the entire feed), the ability for users to block whole instances, and the ability to import/export accounts with other instances (or just back up your account).
Also I'd like to apologize for the downtime; we expected this to be a simple update taking only a few minutes, so I didn't post an alert in advance, but then were some issues that caused us to go offline for several hours.
Lastly, after the update I found I had to clear my browser cache/cookies in order to log in, similarly, users using apps have been reporting they needed to remove and re-add their accounts. You may find you need to do that too!
EDIT: The app Liftoff has a bug preventing login on any Lemmy instance over v0.19.0
EDIT 2: We've updated to 0.19.3, mostly just bugfixes in this one.