I think a couple people are missing a few things in the description of the book. Classes WILL be changed, WOTC says this in their description of the books. They say to make them more balanced.
People here have been saying "I don't see why anyone would get annoyed over all the errata that we already have being put into a book, don't buy it."
But it ISN'T just errata. There ARE rules changes. I've been trying to think of any way that they could change the classes that would make all of them completely compatable. I can't think of one.
I'm guessing they will only change the Bard and Ranger classes. However, I'm virtually positive that they will change them in a significant way. I REALLY doubt that the changes will be listed online anywhere (MAYBE in the SRD, but who knows). It will be like the SWRCRB. There was a revised Jedi Guardian class. I searched online everywhere to see if I could find details of that new class as it is the ONLY thing I wanted from the revised book. But WOTC didn't have that information anywhere. They had a web enhancement on "converting from the old core rulebook" but it was useless without the revised core rulebook in hand.
I'm worried that the new book will include 2 revised classes, a better description of what already exsists, and all the errata so far. To me, since I've read all the FAQ and every Sage Advice from Dragon and read through the whole errata, I don't really need any clarification. On the other hand, I'll need to know the information on the revised classes as those will become the new standard in all later books. So, in essence, I'll be forced to buy the new PHB just for 2 classes.
I assume they will likely do the same thing for both the DMG and MM, they will include at least ONE new thing with each book that you can't find elsewhere. They will try to make it something very important in each one so "people like us" will want to buy it. If asked why we will be required to buy a new book for so little information, they will reply "because we didn't want to change TOO much, so as to keep it as compatable as possible with the old core rulebooks."
Of course, because I love D&D so much, I'll end up buying the new books likely. But it'll mainly be for:
PHB: New classes because no one in my group plays bards at all and no one is a ranger longer than 1 level.
DMG: Updated price lists for magic items and magic item creation rules.
MM: Updated rules on playing monsters as PC (and hope it doesn't completely nulify needing Savage Species)
Majoru Oakheart