Well, it certainly seems to live up to the original mission statement of Essentials: an easier class for beginners to grasp.
The fighter used to be the classic 'easy class,' but in 3e, it started being pretty hard - at least, hard to build and play effectively. In 4e, no class is really /that/ easy (as simplistic as a 1e fighter), but Strikers are probably the least trouble to figure out and play.
The Knight is back to being an easy class. It's still got some inevitable 4e complexity. The qualifiers on the Defender's Aura might have newbies scratching their heads a bit ("What's marked? Why does that stop my Aura from working?"), but that's minor. The basic-attack-enhancing mechanic does make the Knight good at OAs and charging, though.
The inability to swap anything to or from regular fighter builds (beyond Utility Power, since they see to get those) makes it more like a separate class than a sub-class. The lack of daily powers (again, except perhaps for the odd utility) gives the Knight different resource-management issues than other classes, and could lead to marked imbalance depending on the length of adventuring 'days,' the how varied the level of challenge presented by encounter is, and to what degree challenges for the day are 'telegraphed' or 'unpredictable.'
A class with fewer or no daily resource-management issues is favored by a DMing style that tends towards long adventuring days with unpredictable encounters - which forces characters with dailies to hoard or run out of them, giving them many occassions when they can't or won't use their best power. Conversely, if encounters tend to be very difficult and spaced a day or more apart, so you have single-encounter days where daily powers can be used agressively, then classes like the Knight tend to fade into the background as their actions fail to stack up.
Of course, the availability of the Fighter makes that much less problematic than before. In campaigns that clearly favor classes with dailies, Fighters will be played, and the Essentials alternatives ignored.
I must support MrMyth here:
if not so much easier, it makes combat way better:
"I attack the goblin/ I swing my sword at the Goblin" sounds much more like an RPG fighter than:
"I use reaping strike power at the Goblin"
It has everything to do with flavour...
No one appends 'power' to reaping strike like that, they go "Reaping Strike!" I've seen a number of fighters played, and the players always had a blast with the powers, the anime fans in particular litterally shouting out the names in-character.
And, I really don't think "I make my melee *basic* attack against another goblin, again, since that's all I can do..." is all that riveting or flavorful. "Hey, Knight, when are you going to learn an 'intermediate' attack?"
Now, fighter fans have always had the option - and many of us exercised it with great abandon - of at least trying to describe each futilely repetative attack a little more interestingly. Some DMs would even hand out the odd bonus if the attack you described seemed particularly creative or tactically sound.
In 4e, though, we actually get the same breadth, variety and power of options as casters. I know that pissed some folks off, but I don't see a reason to turn the clock back on that, and Essentials isn't doing so, since the existing Fighter builds are always there in all their resource-management-required glory.
