Same problem here on a mid-2010 21.5" iMac. Will read CDs and 1 out of 100 DVDs (1 kids' DVD, weird) but generally nothing in DVD format.
I tried all the solves listed (change prefs, reset EMC and both PRAMs, boot from internet disks, even an Endust cleaning CD). Nada. I then set up a Joint Venture call and they diagnosed the drive as faulty after repeating all of the above. They say it's very possible that dying Superdrives can read CDs and even burn disks but not read DVDs. They said it'd be about $150 to replace the drive at a Genius Bar.
Here's where I try to be helpful.
If you're found this thread and you're not under warranty, don't waste ANY time testing it or trying to fix. Certainly don't roll back to previous software versions -- wasting hours of time and eliminating software features you may like or introducing incompatibilities. Just hop on over to Amazon and buy a $30 (not kidding) Amazon Basics external DVD drive. I ordered one w/ my cleaning disk knowing I'd prob need it and prob wouldn't want to pay Apple to fix my 3-year old machine. Works like an absolute charm for me. Faster ripping CDs than the Superdrive, plays DVDs through DVD Player, VLC, or XBMC instantly.
So my advice (ignore it if you like, it's worth what you paid for it) is to ignore any conspiracy theories about whether Apple planned this. IMHO, it doesn't matter for $30. Your time is worth more than $30. Just buy an external and get on with your life. You can even use it with your drive-less Air when you get one, if you don't have one already. How often can you solve a complex computer problem instantly, and be better off in the end, for $30? Just do it. 🙂 End of advice.
HTH.