How is power strike more fun than encounter powers or dailies?
I don't think its goal was to be more fun - it was to be easier to use for newer players.
I've seen players who, with epic level characters, still rarely use Encounter powers - simply because they revert to At-Will or basics as their default attack. And by the time they've rolled the dice, it is usually too late to ask them if they had wanted to use a stronger power.
With Power Strike, however, that isn't the case. They hit, you can prompt them to use Power Strike, and bam! The enemy takes more damage.
Much more limited than encounter and daily powers, yes. More useful for some players - especially those who outright
want to just play a guy that stabs the opponent a bunch? Yeah, absolutely.
How is less flavor and more shared class features more fun?
I'm not seeing less flavor anywhere.
As for more class features, I think it gives a greater sense of the build growing as you level, rather than just having specific powers ramp up or get replaced by bigger attacks. Even if functionally it all comes out the same, I can see more flavor in looking forward to "Advanced Fighting Style" at level 9 instead of just "Another Daily Power".
How are stances simpler than at wills for new players?
Once a player decides they are in a stance, it is basically just 'on'. For whatever reason, that can be a lot easier to internalize than the concept that you have a basic attack (which is your default attack), as well as At-Will Powers - which are also default attacks that do more stuff but are more limited in use.
Basically, if you are always making Basic Attacks, and simply happen to have it modified in different ways based on your current stance, that can feel more natural for some people than having an array of different options that you need to choose between.
I don't think it will be a big difference for everyone. But I absolutely know players who would find the Essentials approach more intuitive.