My 27 inch iMac's dvd drive is acting a lil nutty. It takes CDs just fine, it burns them just fine, no matter what scratches are on the back but as far as DVDs, thats another story. It's picky on DVDs. I burned one DVD that had scratches & even a food smudge on it (not finger prints) I borrowed & it burned fine...but in other cases, a DVD would have minor scratches..or some scratches & it'll reject it. It's weird.

Not as strange as you think. There are actually two (at least) lasers in disc drives. The data is encoded at different wavelengths and each laser only reads its respective type. CD/DVD/BluRay, etc.

As for the issue you are having, it could be a dirty lens, requiring just a cleaning, or it could be the laser is malfunctioning or dying. A few squirts of compressed air may clean the lens, or a special cd/dvd lens cleaning disc may do a more thorough job. Generally I have found that once they start going though, its time to replace. I use my discs so rarely and have been on disc drive free Apple laptops for so long, that I just picked up a 30 dollar burner and use it even on my iMac (cause I usually just forget the thing has a built in one.

I use the Samsung SE-218CB which actually burns Dual Layer discs faster than the Superdrive anyway. Plus I can use on my Mac Pro at work, or my Retina Macbook or my wife's Macbook Air. (I honestly almost never use it anymore too, after ripping my DVD collection and getting Netflix)