TLDR: new demo with a bunch of new stuff.
Go here for download.
Slightly more detail:
Spoiler:
Changes in demo include most everything I've talked about since the last demo release.
Some stuff I haven't talked about here yet:
-Numerous map tweaks.
-New modules. Most notable is the active probe which stops your lock from decaying. Opens up interesting possibilities for missile mechs.
-Balance changes to existing modules. Weight and speed scaling has been changed, the main point is to make Advanced Actuators more viable.
-Some new UI features, like scaling of radar contacts and a handy new "clear" button in the loadout editor.
-Previously hidden mechs have been made available. These are generally weak/weird designs like the Ant. You can opt into seeing them in the pilot menu. In the demo this will amount to 4 "new" mechs, 2 lights and 2 mediums.