Chrom having a completely different Up-B is really starting to make me wonder why Doc, YL and Tink aren't considered Echoes...
For all Echoes, we also know that appearance, voice/sounds, taunts, victory animations, etc. are different. Some moves may have different animations while having the same hitbox data. Alternate costumes have identical animations and move data.
The only two we know (basically) everything about are Dark Pit and Lucina. We know that changes to Marth's side-b applied to Lucina. Dark Pit and Lucina have the same animations, movement speed, weight, etc. as their source characters. Dark Pit is the same size.
The differences we know for them (assuming no new differences) are:
Dark Pit - totally different Final Smash, differences in speed/damage/knockback on his side-b and neutral-b, and the electric effect. There are a couple small differences on a couple other moves (jab and f-tilt)
Lucina - she's slightly shorter than Marth, and lacks the tipper effect with damage being slightly increased to compensate (i.e. a sourspot Marth hit is weaker than a Lucina hit), she has some minor differences on her jab, u-tilt, and Shield Breaker
So both have a couple minor differences on standard attacks. For Dark Pit you have one totally unique move, and two fairly altered specials, while Lucina has slightly different global attributes (slightly shorter, no tipper).
With Chrom we've seen:
different u-special, although it's borrowed from Ike
different Final Smash, although it seems to function pretty similarly to Marth's Critical Hit
By comparison, Doc has:
different speed and weight (potentially slightly different size)
more damage on most moves, different knockback/trajectory on many, differences in frame data
Tornado instead of FLUDD
Pills instead of Fireballs
unique d-air
And we haven't seen all his moves yet to know if there was more decloning (personally, I think it would be cool if they added viruses to his moveset).
Young Link and Toon Link are both
very different from Link in terms of size, weight and speed attributes, their swords are also different sizes (Although Tink's and Yink's are pretty similar). They also have several unique moves, and Young Link is now even more different from Link due to Breath of the Wild content being incorporated into Link's moveset. Presumably they will also have many differences in damage, knockback, frame data, hitbox placement, projectile speeds and trajectories.
Link's other unique moves/attributes:
Sword beams
item arrows, double arrow shot
dash attack (presumably)
remote Rune Bombs
no Hookshot/Clawshot
Ancient Arrows Final Smash
Tink's:
different n-air, f-air and b-air, d-air is stall-and-fall
different f-smash and u-smash
Triforce Slash Final Smash
Yink's:
Jab combo
Fire arrows
Deku Shield probably still weaker than Hylian Shield
Probably getting Fierce Deity Link as Final Smash
Each Link pair is more different from each other than Dark/Pit and Marth/Lucina are. And that's assuming we don't see more decloning, whereas we can probably assume that decloning for Dark Pit and Lucina will be minimal due to their Echo labels (we've already seen some changes apply to both Marth and Lucina). I'm guessing Tink's new f-smash was motivated by Yink's return, so we may see more changes to either character.
We won't know the limits of Echo differences until we see their full movesets...
), she is THE Metroid Prime.
