Some comparison shots here:
I can't speak to the fx shots, but the live action screenshots show a much more realistic and pleasing color palette to my eye. Look at Spock's jacket, for example, in the Engineering shot. The blu ray reveals the accurate slightly marroon shade of the uniforms. By comparison, the DVD looks like a tv with the color cranked up. And the bridge shot, the bridge walls are restored to a proper neutral grey balanceas well.
Isn't there something going on with digital scrubbing or DNR on these blu-rays? Especially Trek III & IV. The picture seems really off, like it was sliced out and pasted on top. I noticed this when I played them too, but it's clear in these photos.
They used old 1080i transfers that had been designed to be downrezzed for DVD or shown on HDTV. Part of creating those transfers probably involved scrubbing the detail.
Most people think that, if they took the time and resources to do new 1080p transfers from the masters, they would've looked much better on BD. I agree, but I still think the BDs looked much better than the DVDs, and I enjoy them.