I'd say that "everything is core" should be something that guides the design philosophy of
WotC more than it should impact the campaigns of individual DMs. It's partly what TwinBahamut, malraux and Cadfan mentioned: that all released races and classes will get ongoing support.
In addition, and more importantly in my view, it's the idea that all released material should be usable (although not necessarily
used - see quote in sig) in any campaign. If spellscars and swordmages are okay in the Forgotten Realms, they should be okay in Eberron. If dragonmarks and artificers are okay in Eberron, they should be okay in Dragonlance. The players and the DM may have to come up with some story reason why the game element is now present in the setting, but it should be mechanically sound.
I contrast this idea to some of the setting-specific crunch in 2e which could not be easily exported to other campaigns because of balance issues (we knew they were present even though we were less sophisticated in our thinking about balance issues in those days): Dark Sun races and Birthright bloodlines are some of the things that come to mind.
And yet another word from our sponsors
