They claim that SLCD is more battery efficient than S-AMOLED, but real world testing is showing the opposite.
The main advantage is not having pentile subpixel matrix, so the display is clearer (most noticeable when viewing text).
Apparently its viewing angles are better in one direction (vertical I think), and S-AMOLED is better in the opposite. Moot point really, since you wouldn't ever really use your device at angles that acute in either direction in any case.
Its blacks are respectable, although not quite S-AMOLED quality.
Colour reproduction is better, although some people (myself included) prefer the super saturated colours of AMOLED screens
Touch response is probably inferior, since the reduced layers in S-AMOLED is one of the key advances of the touch response available in those screens.
Other differences:
HD has:
More RAM (768MB)
A similar CPU (both based on 45nm tech for battery conservation)
An inferior GPU (41m triangles/sec Vs 90m/sec in the Galaxy S - the former number is direct from Qualcom)
1.5GB of internal memory (? unknown how much free for app installation) Vs 8/16GB in the GS (which has 1.8GB of memory for app installation)
No doubt a better file system and much better speed of operation in general use
Sense UI Vs TouchWiz
No TV Out via cable
An 8mp camera with dual LED flash and 720P video capture Vs 5mp with no flash and 720P video capture (and despite that I'm sure its images and video will almost always be worse than those on the GS, given HTCs history where imaging is concerned, and having looked at the camera samples from their other 8mp handsets).
Less audio and video codec support
Probably a few other differences which aren't springing to mind right this second...