Well, this thread has convinced me of one thing...I may just have to buy my backup copy of the 4Ed PHB1 a lot sooner than planned.
Nah, from the Black Diamond twitter, it sounds like they'll have plenty of copies to come for years. And there's always e-bay (Personally, I just replaced my worn, finally-falling-apart copy of 1E DMG with a new, unused copy*) for less than I bought it in it's heyday.
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On my own side notes for older augmentation to editions; I came into AD&D right about the time UA came out. Paladins got some noticable changes as they became a subclass of cavalier, as well as many hefty racial level changes and allowed class / combo changes. Then, there was the barbarian and many of the spells wizards take for granted got added to the game (like Mage Armor). Most of this stuff had been previously seen in Dragon Magazine. Yeah, the changes were
very noticeable - and more than a mere splatbook's worth. Looking back, it was definitely 1E's .5 edition.
In 2E, the Player's Option was touted with a strong emphasis on "option". Some of it (especially items in Spells & Power) were incompatible in the same game. As far as I can remember only one official module (Firestorm Peak) and one Dungeon Magazine module actually incorporated PO stuff - TSR was very careful to make sure it was not "required". It also attempted to "stealth errata" material, such as kits and psionics. Looking back, it was certainly 2.5, though the approach was nothing like 3.5 - especially in the fact you still needed the original PHB/DMG/MM.
Overall, I get the sense that the essentials line is a stab of sorts at 4.5 in that it is an attempt to incorporate new updates to previously printed core material. Moreso though, it strikes me as the start of a repackaging model, almost as if it were incorporating GW's rules as one book, and classes/races as "codexes" (It's also the model White Wolf uses now for WoD, and Savage Worlds uses for its rules + plot point campaigns). WotC's the "game titan", and like all titans, it takes some time and effort to wind up to what the little guys are doing - but once it gets started, it tends to roll everyone else over (with innovation).
* I'll never play 1E again, but I like to read the old gal from time to time.