Kudos for tackling the tough subject with grace. I'll see how well I can answer this. If anyone thinks I've gotten evangelistic, please let me know and I'll try and modify my answer to reflect your concerns.
What Is Essentials?
There will be several products released this year and early next year under the Essentials banner. Here's a list of the key ones. If you get those, you will have a core set of rules that allows you to easily play and run D&D 4e.
There are few core changes or errata that originate with Essentials. Combat works the same way, magic works the same way, and so on. There have definitely been a lot of errata and rules changes over the last two years, and Essentials will include all of those, but Essentials in and of itself is not the reason for those changes. For better or worse -- and there are arguments in both directions -- WotC decided to aggressively fix balance problems in 4e. That's not new to this release.
The classes presented in Essentials -- fighter, cleric, rogue, wizard, druid, paladin, ranger, and warlock -- are not the same versions as those we've seen so far. In some cases they're pretty similar, and in some cases they're fairly different. However, they are fully compatible with existing characters of those classes, in two key senses.
First, you can take an Essentials wizard and plop her down in a campaign next to a wizard as presented in the Player's Handbook and nothing at all needs to be changed for either of them. The core gameplay is completely compatible, again. D&D 4e is designed so that classes are heavily modular; a given class can have multiple builds which function differently, and one class may gain powers differently than another, but in the end you're just rolling an attack and applying damage and/or conditions.
Second, material from Essentials serves as a resource for existing classes and vice versa. For example, you can choose Essentials wizard powers for your classic wizard without problems. Likewise, an Essentials wizard could take PHB wizard powers. No need to house rule or adjust. In some cases, this won't make sense -- for example, Essentials fighters don't have dailies, so they can't take PHB fighter dailies -- but even in that case an Essentials fighter could take a PHB fighter utility power.
All that said, Essentials is going to feel different. Fighters, for example, work much more like third edition fighters. I suspect some people will want to play Essentials-only games, and I think there's good reasons why you might want to do that. Further, if you learn to play an Essentials fighter, there'd be a slight learning curve if you then wanted to learn how a classic fighter works.
Only Wizards knows for sure, but they've talked about a bunch of things. They want to have a better entry point into the game. The Starter Kit will be sold in Target for $20, and that's a lot more accessible than telling new players they need these three $35 hardcover books.
They also clearly want to attract players of older editions. As noted, the fighter is simpler and does away with some of the immersion problems people have had. The classic 4e fighter is centered around powers, and the Essentials fighter returns to the mode of just using basic attacks -- but doing cooler things with them.
I believe that Wizards would like to have a new core rule set that collects all those errata. I don't find the PHB to be useless... but I also get most of my rules info from my DDI subscription at this point, and I'm really looking forward to the compiled Rules Compendium. It's a pain looking up finer rules points in three or four different places.
What Happens To Old Stuff?
Nothing. There is not going to be anything like the Complete Class line which made the old class books obsolete. Essentials options don't overlap with classic options; if I want to play a two-weapon fighter who gets by on mobility, the Tempest Fighter from Martial Power is probably still the right way to go. The monsters from the first three Monster Manuals are completely functional and don't need to change -- at worst, you might want to up the damage for the ones from MM1 and MM2. The campaign books don't need new editions. Etc.
Most of the books announced for 2011 are not Essentials books; they don't have the Essentials name and they don't have the Essentials trade dress. I believe the intent is that they'll be equally useful whether you have the old core books or the new Essentials books, which makes sense because it's theoretically the same game.
The proof is in the pudding. A few core rules are changing with Essentials. Racial traits are slightly different here and there, for example, but in all cases the changes are optional rather than forced. Magic items get a new rarity scheme, and I don't know exactly how that's going to interact with old magic items.
All the above wall of text is what I think, but I do want to look at the books and find out for myself.