Can't read certain DVDs

Many have reported mixed results reading DVDs on Macs and here are the most common reasons:

1. If these are all commercial movie DVDs, and they are the standard size on
http://docs.info.apple.com/article.html?artnum=58641

and the problem is strictly with playing movies, I'd contact Apple here:
http://www.apple.com/contact/phone_contacts.html

2. There are some programs in the media content of discs that only work on Windows or a special DVD Player called Interactual Player. http://player.interactual.com/default.asp?code=MAC

3. If these are homemade DVDs it gets stickier. Many people burn to DVD something other than the standard Video_TS folders, thus they aren't playable in a standard DVD Players. DVD-R DL is not as supported as DVD+R DL.
Older Macs don't support DVD+R DL or single layer at all. To find out if yours does, check the specs of your Mac:

http://www.info.apple.com/support/applespec.html

If the Mac only supports CD-ROM, CD-RW, or CD-R, then it doesn't support DVD at all. Combo drive Macs support playing DVD-ROM and DVD-R discs, but not DVD-RW or DVD+R unless the specs actually say those formats are supported. Superdrive Macs of varying vintages may support DVD-R, and DVD-RW, but not DVD+R or DVD+RW when it comes to burning.

PC burnt DVDs in UDF or Joliet may not read at all whereas those burnt in ISO 9660 have a much better chance of being read. For UDF, you may need:

http://www.softarch.com/us/products/rdvd.html

4. DVDs burnt for different regions than a region locked DVD drive may also not play:

http://docs.info.apple.com/article.html?artnum=60183

5. The quality of video may be graphics card dependent, dvd burn or may be player dependent. You can verify this by trying a different DVD Player such as http://www.videolan.org/ 's VLC Player. If the playback is identical, then it is a graphics card dependent issue, or the DVD wasn't properly encoded to give the highest video quality possible. If the playback is different, you can optimize the DVD playback quality in Apple's DVD Player software by changing the interlacing setting as described in this article:

http://docs.info.apple.com/article.html?artnum=25318

6. The burn speed of DVDs matters a fair bit. Some older DVD players will only read 1x or 2x DVDs. Check if your DVD drive required these firmware updates:

http://docs.info.apple.com/article.html?artnum=86130

Before attempting to read 4x media on older machines.