My read of the essentials line is this:
1) We [WOTC] had all this material we were going to put into various Power books, which is now going into the Essentials books. For example we had material for Martial Power 3, which would include a variant fighter that used At-Will/Encounter power-boosting instead of daily powers. We had variants of the other classes as well, intended for other Power books. Now all that goes into the Essentials line instead of new Power books.
2) We have all the errata from before, including some new errata for one line of Wizard spells, that will be included in this.
3) We re-formatted some rules and put in some better descriptions of some stuff, to make it easier for new players to pick this stuff up.
4) We put it all in paperback, so it's less expensive to buy.
None of that screams new edition to me, or new half edition either. All of the material is, from my read of it, stuff that would have ended up in expansion books anyway, and in errata anyway, except for some additional formatting and descriptions. But by packaging it in this different way, they pick up some new players (they hope) while still selling to old players (they hope).
It all also seems 100% compatible with existing 4e (with errata), given any changes to existing characters will be in the errata anyway, and are still relatively minor in their nature.
I suppose you can call try and call this 4.5e simply because of a larger amount of errata, but frankly I didn't think 3.5 was just 3.0e with more errata. Some things simply were not compatible between those two sub-editions, and could not just be fixed with some errata.
And yet, with the Essentials line, it does seem like it all works together, and the amount of errata is manageable and within the same rules sub-systems as opposed to a brand new sub-systems.
For example, adding some "miss" descriptors to some Wizard spells is not a new sub-system or new way of handling the Wizard class or their powers. It's a pretty normal add-on that I can see them doing with errata, and I seriously doubt that errata alone would have triggered anyone claiming it was a new edition.
Heck, it seems like a more minor change than the 3.5 polymorph errata they tried to do. That errata changed entire character concepts, some creature concepts, and a whole slew of stuff that had to be torn down and rebuilt almost from scratch. Maybe people called the polymorph errata a new edition too, but I don't recall that happening much.
Tevor Kidd's EW post seemed pretty telling to me:
And seriously, new builds is what I would liken the crunchy bits in the Essentials players books to. You've got the basic PH1 Fighter, you've got the battlerager, and then you'll have the Essentials fighter build. You have the Bow/Two weapon ranger, you have the beast ranger, and you have the Essentials ranger. The rules for playing the game don't change (beyond adding the rules updates into the compendium), and a party could easily have an Essentials build rogue right along side a Brawny Rogue from PH1 - that is, assuming the party wanted two melee strikers.
And the EW News item from a couple days ago:
"This point bears repeating—Aside from rules updates and changes to one category of wizard spells, the character you are playing today does not change in any major way. It was crucial to us that someone playing a dwarf fighter today didn’t need to rebuild that character once the Essentials products were released."
Now, could you honestly say that last paragraph about the changes between 3.0 and 3.5? I know I couldn't. My half-orc fighter was shredded in the changes to 3.5, and the entire concept had to be scrapped due to the changes. 3.5 seemed to require complete character rebuilds for my entire group. It would seem the Essentials line does not require that sort of change, thus making it not a new edition (or sub-edition or whatever you call adding a .X to the name).
The new Cleric article sheds some more light on the issue:
Compatibility: The Essentials products aren’t a new version of the Dungeons & Dragons roleplaying game, though they do give us a chance to clean up and clarify a few things that have been causing issues the past two years. In the end, current players had to endure as few changes as possible. The only changes we embraced were ones that we would have implemented even if we were not producing the Essentials products. That’s why things like the new approach to racial stat modifiers appeared in Player’s Handbook 3 and the higher monster damage appeared in Monster Manual 3. Regardless of the directions the Essentials products took, we wanted to implement these new approaches in the game.
Overall, it sure doesn't seem like a new edition to me. Certainly not to the extent that 3.5 was a new edition from 3.0.