This is absolutely right. The 1st Harry Potter movies looks decidedly dodgy in places (especially in the FX sequences - which is after all most of the movie). It reminds me of the Ray Harryhausen movies, where the grading changes when an FX shot comes in and you can see join. Odd. Indeed, the UK HD disc of Kong doesn't hold up well in FX secenes either (I know the US one's better), so I assume that a combination of the technical and the greedy is what's holding up New Line with LOTR.
Older films use better stock, too. Film stock began to look very grainy in the 70's and it's getting worse now. Video to film to disc transfers are often deeply iffy. Classic 3 strip technicolor still looks absolutely stunning, and I'm surprised that more of these classics don't make an appearance on HD discs. The Adventures of Robin Hood, Singin in the Rain. Movies like that. I know they're 4:3 and they did get out briefly in the US, but there has to be a market for them in the HD universe.
I have The Searchers on BD and it looks stunning, even though the restoration is a little flawed. That's a VistaVision movie, in which there was even MORE detail in the frame, is often true 16:9 and therefore should be a given for an HD makeover. So come on Warners and Paramount - give us North By Northwest, The Court Jester, Vertigo, High Society and Funny Face.
I'd rather see them than an HD version of The Simpsons anyday.