http://gundamguy.blogspot.com/2011/12/gundam-seed-hd-remaster-comparison.html
Beware of spoilers
Besides fixing the "derp" scenes mistakes, there's nothing really significant that will make me rewatch it for just the sake of the HD.
So, if you haven't watched it at all, better go with the HD.
General changes
* People have lesser forehead (Athrun and Murrue specially)
* Clothing now have a brighter color
* Mobile Suits are more accurately drawn having a much more realistic design and articulation. By realistic I mean 3D model and GunPla perspectives
* Shadows are done right
Specific changes
* Athrun has a smaller forehead
* Cockpit hatch looks more detailed
* The GINNs' thruster backpack looks longer
* Strike looks more badass now
-- Did I mention Athrun now has a smaller forehead?