Quality vs Resolution
Thing is, everyone was banging on about the amazing quality of Blu Ray long before many discs were released, but the reality is the technical benefit of the format is just simply that it offers up to twice the resolution of NTSC/PAL, which is not the same thing as twice or indeed any better quality.
The problem is a lot of high def transfers on back catalogue titles are done poorly or use existing transfers that were done for DVD. They may have been originally transferred at a high def resolution, but the encodings were designed for DVD with excessive edge enhancement, contrast boosting and noise reduction (or total lack of where a dodgy old print may require some at least).
Gladiator is a good example, hence the re-issue of the disc following massive complaints of a shockingly bad HD transfer.
Aside from that though, HD has a large perceptual factor. We don't see higher resolutions very well, especially at distance or with a small display. The human eye and brain responds much better to differences in contrast. It's long been a trick of DVD manufacturers, film makers, even photographers or media editors, that boosting contrast and enhancing the edges in images makes an image "pop" and look much sharper and appear to be higher resolution than it actually is. The smaller the image the better too.
A lot of people with HD TVs also have too small a TV to really appreciate the difference. It is possible to *notice* a difference but appreciating it is harder. There is also a percentage of the population who really couldn't care anyway. They appreciate the film, not the definition of the picture. There are a lot of these people, which is why iPods are popular, and people are quite happy watching terrible pictures on awful screens on a plane, and won't bother watching the same film again because they've "seen it on the plane" despite the crap quality.
Myself I rarely buy Blu Ray anyway, but when I do I wait for something I really want and will appreciate (gone are the DVD days when I'd buy anything released and only watch the thing once), and having done much research to ensure it's a decent transfer. Otherwise I'm wasting my money on something that will be re-released or is available on an HD channel on TV or will be on an HD VoD stream some day (not far off now). Blu Ray to me is a stop gap and not the future. Discs have had their time.