Glaring flaws. Like what?
Okay, first the whole
Remember the New Guy. That one was obvious, and ripples through pretty much the entire episode, ruining much of the good stuff. And really, it made no sense whatsoever. Not being able to tell your sister in person is one thing, but surely he could spare a few minutes to dictate a letter?
Then there were the actual main characters being completely sidelined, and acting rather OOC. As mentioned before, Applejack wouldn't suffer anyone dissing her treats without comment, for example. The fact that no one seems to listen to Twilight despite Candence's behaviour is just weird. They have been best friends with Twilight for a long time now, and no amount of 'wedding nerves' could justify the extend of the bride's rudeness.
The resolution felt lacking in impact. Partially because it revolved around those two new people, partially because it was too sudden. You can't have something cheesy like that with characters that we don't know. (I am suddenly entertaining the idea that I'd rather have SA punch the Queen in the face, or something. Less supernatural cheese, more mundane awesome)
It did a whole lot of good, but its ambitions reached further than 44 minutes would allow. The show runners should have kept that in mind when they wrote this, and I consider that a fault.