Going by just the first three corebooks...doesn't Essentials change skills (stealth), change skill DC's, change the math behind monsters, change powers and drastically modify classes as seen in the previews... oh yeah and also get rid of warlords?
No. Stealth has already been changed (months and months ago), ditto with Skill DCs, the monster math has been changed as of MM3 (before Essentials).
All of those changes are already known by players and incorporated into their games - if they want to. Indeed, PH2 has the Stealth changes, DMG2 the skill DCs, etc.
Essentials haven't changed any of those things. It does provide a good time for the non-power updates to be reprinted in the Rules Compendium, though. (The hardcopy, not the DDi version).
The one thing Essentials does change (although it will be available to everyone via the Rule Updates) is the way Wizard Encounter powers work... they'll have their regular power and then gain a Miss effect.
Essentials *does* provide alternative builds for many of the core classes. Doesn't invalidate any of the old classes... and future books will have "essentials-style" classes and "PHB-style" classes in them. (Indeed, two takes on each class in each book).
Warlords aren't in the essentials books. So what? Neither was the bard.
Cheers!