Back in the VHS days any anmated theatrical feature was considered part of the Gold Collection.  Currently they leave out title that are half live action like Mary Poppins, Pete Dragon, So Dear To My Heat and Bedknobs and Broomsticks.  They also leave out Pixar movies like Finding Nemo and Toy Story.  They leave out chepquals like Aladen 2, which I agree with.  This is the one that I don't understand, they leave out some of there good titles like Hercules and The Many Adventures Of Winne The Pooh.  They used to include everthing but the direct to video sequals.  Are they trying to make The Diamund Collection very exclusive and only include there top notch titles.  It may be way to warly to tell, but does anybody have any speculations?

As Kinoniki said, The Diamond Collection is the replacement to the Platinum Collection. The gold collection, or later special editions, are everything else.

The Diamond Collection only consists of a certain level of tiltes, I believe it's 12 titles right now and I think it's based on the sales. For instance Aladdin is no longer a Platinum title because the dvd sales did not meet the requirement.

That's probably a very basic explanation but I think it covers the gist of it. Probably can get more info from the diamond and\or platinum collection entry on wikipedia

-Agrare