The main push with this series of updates was to get many more (30 or 40 more in fact) new community made designs into the game to represent the different factions of the game. You can see each faction in more detail here
A number of bugs have also been fixed and the main show stoppers in multiplayer have been fixed as well.
[bug fix] structures and fortresses now work in multiplayer
[bug fix] Password guess box is added when server is password protected
[bug fix] laser (simple) description updated to reflect that it uses ammo
[bug fix] capslock vehicle designer vehicle hold no longer slows down game for big vehicles
[bug fix] respawn beacon added to starting fortress, as well as some spare part crates
[bug fix] Saving vehicles in multiplayer no longer makes them 'enemy spawn' when loaded again
[bug fix] Precision spin block logic tightened up- they rotate in the same direction as normal spin blocks and their speed is correctly calculated.
[bug fix] bug fixes in the visualisation of prefabs before placement
[bug fix]Fortress controller moves fortress in orientation of fortress, not the controller placement
[bug fix]Shrinking blocks bug fixed for drills/laser components
[bug fix]Setting the rotation angle of a spin block via automatic control block does no longer revert 'rotation reverse' to 'rotation'
[bug fix]Some issues fixed with the selection of a vehicle of interest by looking at it
[bug fix]large block placement code fixed to allow, for example, jet engines to be placed more naturally
[bug fix]ammo,fuel,spares, reserve limits are saved to the constructable
[bug fix]Scuttlebot death in MP is fixed
You can now select the team when spawning a vehicle in the designer
Vehicle designer freeze vehicle button (CAPS LOCK by default) is customisable (CAPS LOCK does not work on MAC in Unity)
Many in game vehicles added and many updated, each faction now has multiple different unique units.
Custom missile staggered fire is now more robust for second salvo
Wood can be placed at multiple orientations, as it could be before 1.53
You will save the vehicle you are building on, when in build mode, regardless of the vehicle you are actually standing on or closest to.
The firing delay for weapons is now enabled
Explosive missile warheads now stack into a single larger explosion
There is a configurable delay on the automatic control block trigger pulse
Changes to the way blueprints spawn in campaign mode
See here for more info