These days, I definitely think Cadance should have been a unicorn as Faust originally intended. Back before S3, I figured that the new alicorn was good enough to be worth it. That her first episode was so great, everything else could be forgiven.

But it turns out since then that S2's finale was some kind of fluke. Not a single episode since then has gotten anywhere close. The Crystal Empire had a lot of potential for being an amazing new place, and if they'd simply fleshed out the backstory and characters more, perhaps it would have been better.

I remember how I originally speculated the Crystal Empire to be the source of the Elements of Harmony. The early teaser art featured a lot of hexagonal crystal motifs, I figured the Crystal Empire might have been the place where the Elements were made or something. It'd even explain why no one seems to know much about the Elements; because the nation that created them disappeared a thousand years ago. Atlantis' lost technology sort of thing.

Would that have been cool? Yes. Would it have added a lot of depth and continuity to the story, and given the Crystal Empire a place to 'fit in' to things? Totally.

Did they do anything even close to that? Hell no. None of the episodes involving the Crystal Empire have been worth much of anything lore-wise.

Bah. Saddle Arabia's horses were worth more to me than the Crystal Ponies.

The primary reason (I think) people didn’t complain, was because she was immediately overshadowed by Chrysalis’ infinitely more interesting character, and promptly forgotten. If the royal wedding HADN’T involved Chrysalis, things would have turned out a lot different.

It helped that she got one of the most amazing musical pieces in the entire show, too. This Day Aria was superb, no doubt about it.

Also, it might have also helped that people still had complete faith in the show and loyalty to the artistic choices of the staff, due to S2 being nearly flawless and lacking in significant controversy. People were willing to overlook a weak character because of every other great thing we got during that season.

Not to mention, as you said, Cadance and Shining Armor were primarily self-contained. They were both new characters who lived in another city, who weren't stepping on the toes of any of our main characters. Thus, they were easy enough to ignore if you didn't like them.

But then S3 happened, having only 13 episodes total where four (five, if you include both parts of the opening) took place in the Crystal Empire. All of a sudden, these side-characters were being practically forced down people's throats weather you liked them or not. Didn't help when the initially promising Crystal Empire turned out to be rather uninteresting (the place didn't really do anything special, when it came down to it), and housed some of the most lackluster episodes of the season.