Classes that can suddenly fill other roles:
Fighter, Ranger, Druid
Eh. That doesn't change existing rangers, druids, or fighters in any way, shape, or form. Yeah, they added new builds for these classes that actually fill different roles. But that still falls under options, not changing the existing gameplay.
Multi power source classes!
Multi role classes!
I'll give you these, but it's not like these are OMG things. It opens up some options for the new builds (like taking Second Skin for Something | Primal classes), but beyond that... eh. Power Sources mostly function as window dressing, anyway.
Different starting proficiencies (cleric vs warpriest)
Different proficiencies for different builds is nothing new. The Warlord can give up certain proficiencies for Archer Warlord, the Fighter has the Arena Training (more proficiencies) option, and Ruthless Ruffian rogues gain mace and club proficiencies (and sneak attack).
In other words, nothing new.
Add all the slow changes they made in preparation of essentials (like many people pointed out they made in 3.5 with sourcebooks like 'Tome of Battle'), like being able to choose between race ability bonuses.
I had missed the ability score changes, but to say these were made 'in preparation for' Essentials is slightly disingenuous. The first versatile stats were those of the Changeling, introduced way back in the EPG.
Also, note that all these changes made sure that the old stat arrays were still perfectly legal, meaning that while some of your old characters might not have optimal choices anymore (Elven Wizards, Dwarven Fighters, etc.), they still work.
Yes, many things are listed as options, but like many of the new feats, showing a better, more supported option (nerf to the feat that allows to change the ability used for melee basic attacks -> new classes get it as class ability; new expertise feats) is also a slow replacement.
The nerf to Melee Training is because of the Knight and the Slayer, so they couldn't take one feat to drastically change their class balance. The Thief and the Scout do not have this problem, since they were build with Dexterity as primary statistic in mind (neither adds it as a static bonus to damage rolls, for instance).
See also the fluff support between standard infernal warlocks and infernal hexblades with their 'iconic' race: The new tiefling can get 'perfect' stats for the essential variant, but not the old 4e.
While I, too, would've liked to see an Int/Con race, and the Tiefling looked like a good fit, they already have the feat support to be very capable Infernal Warlocks, and refitting the Warlock to a class with one single central attribute is much better from a design standpoint.
And I'm fine with it. I'm not fine with people who treat other like being dumb for comparing essentials with a revised edition. It is to similar.
I disagree. I think the idea of '4.X' edition is, to state it lamely, 'so last decade'. With the constant stream of updates and errata, the rules are almost in a permanent state of flux, albeit with a rather stable central core. If Essentials is 4.5, then every update from release date until now was some version of 4.XY, with X < 5.
Whether or not you like this constant change, is of course entirely up to you.
EDIT: Addition:
It is more like Diablo 2 plus LoD than Diablo 3.
For some reason, I actually really like this comparison. LoD followed a stream of patches that fixed bugs, and while it fixed more bugs by itself, but provided mostly added options and possibilities that don't invalidate old characters. Well done

.