Thank you, I'm not sure how I managed to bungle this.I'm going to move this over into the proper forum...
Mechanically, the characters were entirely different (sorry [MENTION=37579]Jester David[/MENTION]). They entirely went away from the AEDU framework where each character was designed the same with the same number of powers. They had an entirely different mechanical feel, and that lead to some play differences as well.
Now, they were numbers-compatible with the earlier characters - it was the same edition so they could co-exist with other characters and run the same adventures. I seem to remember that some may have dealt better or worse if you changed up the number of encounters per day a lot, but that was a lesser quibble.
The classes were streamlined. Some say that they were more like earlier editions to try to woo the players who were still playing 3.5 or switched to PF, and in some ways they were because they didn't follow AEDU, but they were still the heavily tactical 4e type of character.
The thing is, even before Essentials they were changing that. The psionic classes in PHB3 had a different number of powers. Which also played very differently than the traditional classes.Mechanically, the characters were entirely different (sorry [MENTION=37579]Jester David[/MENTION]). They entirely went away from the AEDU framework where each character was designed the same with the same number of powers. They had an entirely different mechanical feel, and that lead to some play differences as well.
You're going to get a lot of answers about redefining class power structures, different approach to design for each class, and better math balance. But what you really need to understand is that Essentials was a mistake. It was the company back peddling from its original design goals trying to appease and regain a section of the playerbase that wasn't subscribing to their new approach. But by introducing this radical change in direction, they only managed to confuse, frustrate, and divide the small following that they had. That is not to say that Essentials wasn't good or had any merit, but like everything else at the time, they just expected us to accept it and cram it into the existing game. It was the beginning of the end, or maybe the "hail mary" pass at the end.
That said, I would definitely use Essentials exclusively as a "Basic" version of the game today for friends and family who are more casual players, and reserve 4e Core for the more hardcore table. Either way, still my edition of choice. But like the peas and gravy on my plate, I like to keep them separate.