Can you play 4E without the core books if you buy the essentials line? I don't mean get by, but I mean will you have all the rules you need? If you wanted to host an official RPGA game at a convention, with a bunch of picky anal nerds, would you be kosher?
With Essentials, you have everything you need to play Essentials. It's a complete game of 10 products, only 4 (IIRC? 5?) of which are actual books. If you want to run a game with /just/ Essentials, but allow players to paly other stuff, you'll have the basic mechanics available for reference, but you'll have to depend on the player's character sheets/cards/books/whatever for the specific rules on all the crazy stuff they have - 'complete & legal' character builder sheets are good for this.
If these are just extra class options, why isn't this martial power 3 or something, or players handbook 4?
Marketing. The ______ Power
n books were starting to get on peoples nerves, just because there were so many of them (and other suplements) coming out too fast. That's been a problem since 2e, but it's a problem that WotC litterally /can't/ fix without cutting their own throats, so the solution to "too many books, coming out too fast" is, of course, more even books coming out even faster. But, with a different name, a lower price point, and a vague promise that you don't /have/ to buy anything more (of course, you never /have/ to buy anything from them, at all).
Is the "essentials" name inaccurate? If you were going to give these an honest name what would it be? 4.5, extras, PH4, (martial, primal, divine, arcane) version x, replacement classes, or something else?
It's accurate enough for part of what it represents, from a marketing/distribution perspective: it's 10 products that WotC recommends retailers stock, and that will 'always' be in print ('evergreen'), in that sense, it's 'essential,' to presenting the D&D brand at your FLGS or local B&N or whatever. I don't know what would have been more accurate - Commemorative D&D maybe, because of the nostalgia-heaviness, exempllified by the retro Red Box?
Are these just replacement classes? If you use the essentials line, do you not use the old wizard, fighter etc?
None of them are officially replacement classes. A few /could/ end up being replacement classes, it depends on how the line is handled in the years to come. D&D has always had a lot of 'power inflation' - that is, stuff that comes out later in an ed is usually a bit more powerful that stuff that came out earlier. 4e was no different, but the fact that so much new stuff tended to be new 'builds' for existing classes kept all classes viable. The only troublesome classes were those with 'split' primaries (like the Paladin - some powers STR, others CHA), if new builds tended more towards one stat than another, the corresponding old build could be left behind. That happened to the Cleric, which got mostly WIS builds, and the STR Cleric went from being pretty impressive when the PH first hit to a likely candidate for euthanasia, today.
What Essentials has done is to inflict a similar issue upon the Rogue and Fighter (maybe more later). The Essentials builds for those classes have no attack powers. At all - they just enhance basic attacks with class features. That means that material for these new builds is virtually useless to old builds, and vice-versa. So, if, post-Essentials, the basic-attack-spammer builds are more or less abandonned, and new martial builds with attack powers continue to come out, the 'new' classes will slowly get left behind, relegated to 'training wheels' builds that are viable only in Essentials-only beginner games. Conversely, if support shifts to the Essentials-style builds, the 4e builds will languish, fall behind the power inflation curve, and eventually become obsolute - and, yes, the Essentials will thus have 'replaced' them.
Time will tell.
So say you own the whole line of books. You have a player that wants to be a wizard. Do you even show him the wizard in the PH? Would you only use the old PHs for classes not covered by the essentials line? Would they be weaker or out of date somehow?
The 4e wizard builds can choose Essentials 'Mage' powers, and, some Wizard powers have been updated to bring them up to snuff for the mage, as well. So both remain viable options. It seem like the Mage has a few things over on the old Implement-mastering wizards, though, like a strictly superior spell book, a bonus at-will (Magic Missle), and a bonus Encounter 'cantrip' that seems pretty good - and more goodies to come. Similarly, the Warpriest and Sunpriest are both WIS builds, so 4e WIS builds can take their powers and vice-versa. Their class features are different, but the WIS-based Cleric builds hold their own, I think. The STR Cleric, OTOH, has probably been effectively replaced (by the STR-based Rune Priest, if not also by the WIS-based Warpriest).
Could you imagine this wizard maybe wanting to be a generic PH wizard even though he has the essentials option? Is the new wizard a choice or a replacement?
The Essentials Mage is, if anything, a bit more generic and magic-usery than the 4e Wizard. So, the Mage is more new choice than replacement.
With past incarnations of the game there's always a point at which the new material coming out seems unnecessary to me. I like shiny new things, but what I like more is having a complete system. It seems that 4e has come to that point with me with this essential line. It's as if the essentials name should really be "non-essentials", with everything up to that point being actually essential.
Maybe you could think of it as 4e (take one), Essentials, and post-Essentials. Essentials is useable as an 'essential,' complete system. 4e is useable as a complete system. Post-Essentials materials would be the 'extra stuff' you can take or leave.