The production of fullscreen versions are being cut down since all TVs being sold now are widescreen. Fullscreen versions are formatted to fit the regular TV ratio 4:3. Some movies are shot originally at 2:35 ratio or 1:85. The wider the ratio, the more details you see from side to side which movies are supposed to be viewed.
Some samples:
http://img-nex.theonering.net/images/scrapbook/orig/8465_orig.jpg
http://farm1.static.flickr.com/225/505973356_952f1561ae.jpg
http://farm1.static.flickr.com/176/386545147_0cc026f999.jpg
http://www.slashfilm.com/wp/wp-content/images/widescreenfullscreen.jpg
EDIT: if your dvd player has a zoom function you could zoom in so it fills the whole screen but that's what fullscreen really is, the original picture gets zoomed in and cropped so it fits the almost box size format of standard TVs. Also, directors can express more impact in a wide shot and you miss that interpretation in fullscreen. Widescreen is more of what you see in real life, your eyes could see everything as a whole, you focus on what's in front of you but you are still receiving images from all angles.
Even if a TV is widescreen, the TV is at 1:85, but if a movie's ratio is 2:35, there would be black bars on top and bottom because the ratio is even wider. What I wanna say is Widescreen is the way to go. Fullscreens were produced only to fit the almost "square" ratio of standard TVs. No black bars but less detail, cropped and zoomed.
EDIT 2: if you see the blackbars on the portable dvd, that means the movie ratio is wider than the screen. If you look at the back of the dvd case, it should say the ratio, 2:35 or 2:40 that means it's much wider. 1:85, 1:78 that usually fills the screen. So it will depend on the movie. If you watch a fullscreen dvd on the portable dvd, then the bars would be on the left and right giving you a more "square" screen. I noticed that most comedy or drama have the 1:85 ratio, more action/special effects have the wider 2:35. If you watch the 2:35 on standard TV the black bars are bigger. But that doesn't mean you lose picture on top and bottom, it's just that the standard TV is now "widescreened". The black bars are needed to show it. It's all technical stuff but welcome to the 2010's! :P Anyway, the quality is really in the movie itself. If it has a good story, great characters, hits the spot for you. Aspect ratios don't matter much. Just remember 2:35 ratio means you'll always have black bars even if you have a widescreen TV.