Im not an expert here, but I think a Blu-Ray disc has about 10 times the space like a normal DVD. So it can display a much better quality.
Because the better the video quality the more space does the video need.
You would never get a High Def movie on a regular DVD but on a Blu-Ray disc.
Ehh.. a BluRay disc can hold plenty, sure. But it's how that space is utilized that effects the end product. For instance.. music CD's AKA red-book audio is extremely wasteful. You can encode a lossless (read

erfect) copy of the same CD and fit it onto a 700 meg CD 2-4 times with the right encoding/format.
The same can be said for DVD, in a way. DVD-9's can hold a lotta stuff, but comparing MPEG2 to the x264 formats used in modern high-def shows and movies is the same as red book versus FLAC.
So much of the space in a retail DVD movie is ******. How many times are we going to listen to the commentary track? That could be cut from the disk to gain 5 to 10% better quality all around. (and offered as a free download via internet-enabled BluRay/DVD players, via a small hard drive to store it on) Same for second and third languages. Dont forget that oftentimes they'll put a 5.1 encoding on there AAAAANNND dolby stereo. Only 2 channels of data, but that still eats up space. THEN you gotta consider all the bullshit special features that we honestly dont really like these days.
Behind the scenes stuff, trailers and documentaries.. in the days of VHS tapes that kinda shtuff was rare and interesting. But when they put such things out for even the lamest, shittest.. complete waste of time and money movies/shows!!!
So sure, BluRay can hold tons of stuff. The real question is.. are they going to say 'oh fuck it, 25 mbps video throughput is enough!' Let's shove every last documentary, interview and propoganda associated with this movie onto the special features list!! Better yet, let's put 30 trailers on it.. and some secret **** that less than 10% of all the people who buy it will ever see! Maybe a full version of 'To **** a Mockingturd' as well.
In other words.. I have little faith in the companies when you consider quality is secondary to quantity all the time with DVD's, LaserDiscs.. etc.