The "floating composite" thing you're talking about sounds like frozen grain to me, and it's what happens when grain-scrubbing goes even more wrong than it inherently is. That's probably new to the HD masters used for Blu-rays, but even if it's not, it's probably more apparent in HD due to the resolution. Definitely preventable error if that's what you're talking about. But there's also some actual film compositing done for VFX which was less-than-perfect, and Blu-ray reveals those flaws too, and that's not a problem with the Blu-ray.
The image quality changing from scene to scene is often the result of optical effects and zooms and is part of the analogue filmmaking process. Again, not a problem with the Blu-ray--although, to be fair, Blu-ray is usually based on earlier-generation film elements than projection prints, so theatrical presentations may not have had all the detail of the Blu-ray.
If you're interested, you can get the benefits of the Blu-ray (audio, progressive video) without getting the hi-res image which reveals some of these flaws in the source material. You can (or at least I can) set your Blu-ray player to output 720p or 480p, which would flatten out the quality differences to lowest-common-denominator resolution like DVD does. Not that I'd recommend this for every title, but I could see how it might help in a hypothetical scenario with a particularly jarring transition.