It's a combination of things.
Some discs, both BD and standard DVD, have all the grain digitally scrubbed out of them. Do a search for comparisons of "Predator: Ultimate Hunter" edition (in the blue packaging) vs the original release with the black artwork.
It also depends what size screen you have. Anything below the mid 40's and the image isn't getting stretched as much, so it will naturally look more intense.
Another factor has to do with the 120Hz and higher refresh rates on the newer HD TVs. Since its inception, TV refresh rate has been 60Hz - 60 lines drawn down the screen every second. With 120Hz (and 240, and 480, and so on), more lines are being digitally created to go in between the existing ones, so now you're getting more detail and resolution than is actually on the disc.
And yes, it is "more" detail because it has to be drawn in the same amount of time. If it was just a "faster" refresh rate, the film would only run 1 hour instead of 2 and it would look like you're watching it on fast forward.
The first time I noticed the difference was watching TPM on Spike on my sister's new 120Hz set a year or so ago. It looked fake, like a fan film shot on a store bought video camera. I tried to explain the difference to her but she didn't get it. Then a couple of weeks later, a neighbor of hers experienced the same thing and described it as "too clear". Then she got it. She's not a videophile so she doesn't really care one way or the other as long as the TV works, but she understood the difference in what I saw.
So go back and check the specs on your equipment and see what some menu options are set to. If you have a 120Hz+ TV, you might have a menu option to force the set to run at the standard 60Hz. Set that and see if the "video" look goes away.
That being said, yes, certain things don't need HD. I don't need to watch the local news in 1080p HD 5.1 surround sound. Reality though is that we're in a transition phase right now - studios across the board are upgrading to HD equipment so there will eventually come a point where what's "high def" now will be "standard def" in the future when an "even higher def" is invented. It's a continuous cycle.