Most importnant changes:
Subs can now intercept TFs as they travel through hexes rather than requiring the to land in the same hex as teh tf at the end of movement.
Level bombers now require far more experience to be effective in naval attack. This is amalgamated with a general reduction in their capacity to hit ships anyway. It is not across the board however as ships that are already hit or docked (unloading or just docked in port) can still be hit. I read this is ships with high speed are almost immune to level bombers (especially with less than 65 exp); so transports are still able to be hit in open ocean but you can feel a little unlucky each time a level bomber hits warships in the open.
Level bombers are no longer anti-fighter weapons. Lower moral fighters now tend to break off rather than get shot down. Level bombers are still very hard to shoot down, but injuries incured in a fight can put paid to them on longer flights (especially if the crews are high in fatigue: >30)
Bombers still magically know about the amount of CAP over a target, now it is more prolific. Bombers require more escorts in order to attack a well defended target. This tends to stop the suicidal bomber only attacks against Port targets that were well defended but has now conversely stopped the bombers form attacking at all against such targets as carrier Tfs (with reasonable CAP).
You can now watch the aerial combat and glean quite a bit of info from it. Planes getting on each others tails, planes climbing to get to bombers or diving to get to lower flying planes. Attacks on bombers at different ranges and the repluies from crew as tehy get closer. It takes a long time to watch a long battle but it is all there (with a little imagination
Torpedoes now are supposed to do more floatation damage than the massive amounts of SYS dmage they used to do.
Support units, such as HQs base units and CD units now put 10% of the numbers into assault strength on defence. This was to offset the ability a sub load worth of troops had to take 10,000 men looking after the aircraft at a base.
Japanese have 50% less ASW ability.
Gawd there are heaps more - someone fill the blanks??
... except if of course you like to hear the sound of torpedoes hitting your CVs