Some of the major and more noticeable changes coming are clearer indicators about what you're being hit by. For example, the color displayed by the damage value has indicated what type of damage you're being hit by, for example:
Red = Normal
Green = Elemental
Yellow = Explosive
But then the game noted direct damage (contact damage with an enemy) and trap damage, which isn't clearly displayed when taking damage. So now we have:
The hard zigzag means you were hit by direct contact, as shown on the left.
On the right, a straight line is used for trap damage, as it's easier to see at a glance when these things happen. The testers felt it would be hard to see the difference between the two during gameplay and I agree.
The hit indicators scale with the size of the damage displayed and will also match the color. (damage type)
As stated previously, Spring Ball will be obtainable in Gate 5, and as early as just after finding the standard missiles. The post-gate terminal will of course lead you to which sector it's in. This of course means pressing jump no longer removes you from morph ball state.
Some other little things:
Air dashing after jumping on a bumper (the big Yoshi's Island arrow springs) will now completely cancel all the momentum gained from the bumper just like double jumping already does. Currently, you can air dash and immediately after the air dash has ended, you'll still rise.
A few more scenes are skippable, as well as the end credits. Many will want to (and you'll want to, trust me) see the revamped epilogue, so it's reachable much much faster.
Which brings me to the end sequence. You can no longer game over on that sequence unless running out of time. (Which no one does.) Falling off the bottom of the screen will now teleport the player to a nearby stable platform.
There's plenty of other changes and QoL fixes coming, plus of course some big surprises that I've not so subtly been hinting at since around April. (Just don't bother asking about it, cause I won't comment on it. :3)