First, and perhaps most obvious, is the design of the straps across his chest. They cover a slightly larger area. It also reveals part of his abdomen, whereas the original one only shows his pectorals. Cause, he wears a massive Shin Ra Electric Company brand SOLDIER belt. You can tell. It's got the logo.
Next is the hair. In the original, his hair is more static at the top, more angular. It invokes images of Aerith's hair from the original, whereas the redesign of both makes their hair look less similar. For.... I imagine reasons.
The space between the straps on his left boot are further apart in the redesign.
The tsuba of his blade is rounder in the redesign, and more square/rectangular in the original. The redesign's tsuba is also gold, the original black.
The original is decidedly more muscular, showing pronounced features near the collarbones, the pectorals, and even the width of the shoulders.
There are clasps on his cloak in the redesign. The original had one buckle. There is a difference between a clasp and a buckle. Yes, there is. I don't care that you think I'm wrong, I'm not, go look it up.
The redesign lacks his massive Shin Ra Electric Company brand SOLDIER belt. In case that wasn't clear enough before, I figure I'd just say it again.
There has been a shift in the triangular points for Materia in the redesign's shoulder pads. It no longer makes an upward equilateral triangle. It is now tilted in a direction (though still equilateral, so the direction is your choice).
The jacket has a more relaxed fit in the redesign. The original is more form fitting to the waist.
The size of the eyes has changed. The original had larger eyes.
The sleeves of his jacket have changed from the original's, which had large amounts of leather above the elbow, which then became more form fitting at the joint and below. The redesign is more relaxed.
The original has him holding his blade to the side, edge up. The redesign has him holding his blade forward, in a guard, edge down. Mind you, edge down is not Sephiroth's style, as he raises and holds it perpendicular to his body, edge up, when he takes his fighting stance.
His boots taper downwards more rapidly in the redesign. The original had an abrupt taper, though not very angular.
The collar of the original's jacket folds outwards, not inwards (see the redesign).
The redesign has a body shape that... isn't disturbing. I mean, look at where the knees are on the original. That's not normal. But, neither is Sephiroth. So, it made sense. The redesign also has a rounder chin than the original.
Lastly, the original design evokes a sense of fire and blood, the red tinge that surrounds him being something that feels ominous, yet shows him eerily above it all, as all the red is underneath him, essentially. As if he stands over the flames, or looks over the pools of blood. The redesign lacks this. How tragic.