Neonchameleon
Legend
1) The difference is great enough to cause a cognitive dissonance that bug some people while playing. Some people can get past it, others can't, especially if they really liked O4E.
That claim alone is enough to tell me you appear to not be reading the thread. A repeatedly made and never accepted challenge has been to find people who have had problems mixing essentials and non-essentials in actual play. There have been plenty of people claiming that because the classes are different they are incompatable, but all the tabletop experience posted has said it's fine. So either you are the first person to be bugged by it while playing (are you? This would be interesting. And new.) Or you are making things up about the thread.
3) Some find Essentials boring. For me, this is my biggest hangup. Conceptually, I love the martial classes. In practice, most anything but the Wizard or classes in some way "power heavy" have been boring to play in all editions of D&D. Sure, you can work outside the rules and find ways to have fun in non-supported ways, but the core combat mechanic doesn't even need human intervention. For example, as a Fighter, especially in early levels, I can simply place a sign on my chair stating, "I attack the closest enemy," and go watch a movie and the tactical ability of the party won't be significantly diminished. There are very few actual choices to be made in combat. 3E is slightly better, but not by much.
We've been through this on this thread. Several times. I'm not going to ever want to play a Slayer. And neither are you. But there are people that like simple classes with restricted tactical and character options. And who get overwhelmed with analysis paralysis with complex characters and lots of tactical options. Others would rather autopilot the combats and star the rest of the time - or just be given something to do while hanging out with friends and don't want it to take up too much of their attention. I'm going to repeat the spirit of a challenge I made earlier in this thread. Why is it important to your fun that these people do not get classes to suit them?
The irony about Monopoly is that I have never once seen or heard of anyone ever playing it by the rules.
Believe it or not my family did. With my father checking the rulebook every game. But I can't think of another case.