Highlights include Sliding Sync (instant login/launch/sync), Native OIDC (industry-standard authentication), Native Group VoIP (end-to-end encrypted large-scale voice & video conferencing) and Faster Joins (lazy-loading room state when your server joins a room).
Does there tend to be logical groups for Matrix channels/servers? (Meaning you don’t really just join Matrix, you go to a Matrix instance for Jerjoba support, or other common interest like Hockey?)
There do tend to be in logical groups, e.g. you’ll see all of Mozilla’s stuff in their server - but there are quite a few general purpose servers.
Matrix has a concept of “spaces” for grouping people and rooms into a unit.
I’m not the best person to ask as I just chat on a few channels in a single server.
There definitely are software projects that run their real-time support through Matrix in the same way others do it through IRC or Discord.
At the same time most servers seem to have a General room (or similar) for off-topic chats.
Peruse the big list of public rooms here. That might give you a sense of it.