Well just from my testing on the test server here are some of the major/minor changes.
Ships ALL ships have 1 durability.
All ships have greatly reduced armor hitpoints and less mast hitpoints making it easier to remove the mast on ships and cause sail damage, along with structure damage to sink it.
Splintering damage is reduced, BUT greatly dependent on the type of gun doing the shooting. (carronades live up to their name of slammers).
Repairs now repair the same amount regardless of the vessel. All repair kits repair 500hp of armor and structure over 90sec. (So on small ships this is a HUGE amount but on larger ships this is barely noticeable.
Sail repairs are the same as repair kits... and act the exact same.
Crew Medkits, can be used in battle, BUT only restore so many men during the fight, not all at once, it is largely dependent on the number of casualties vs the total crew for the ship. But in OW they replace 10men per kit.
Engagement circles are not MUCH smaller, closer to 1/3rd of the previous (current live) version.
Tag timer is reduced to 15seconds.
Towers now have a shorter engagement range. Round towers will only really fire at you when you are within 750m.