I am aware its to be tacked on to "Regular 4e" (whatever that is) however I have no idea why its considered a different THING than 4e. If Essentials is just supposed to be more options but still part of 4e I don't understand the need to brand it differently. Why not call it 4e: Players Handbook 4, or 4e: Simplified Classes or somesuch.
The thing is, it's both. It is (1) a self-contained RPG that a group can pick up and play without referencing anything outside of its books, and (2) an add-on for existing 4e games that can be used alongside existing options. I'm using it as the latter, but I understand there are several groups already using it as the former, as well.
I am also confused at how many of the discussed rules changes (i.e. Magic Item Rarity) fit in with 4e or Essentials.
Well, most of the rule changes have trickled in over the months preceding its release.
Magic item rarity is one of those things you can easily ignore or incorporate. As it stands....
(1) A few items are Common; these are listed in one of the various updates. These can be created by rituals and sell for 20%.
(2) Pretty much every single other item is Uncommon. It can't be created by rituals, and sells for 50%.
(3) Theoretically there are Rare items, but I haven't seen too much of them.
(4) The magic item daily use limit is no more.
Again, though, this has all been pushed out in the various update documents, but it's far from completely clear.
-O