I've always liked the "fallen disciple" trope, so I quite like the idea of Sunset Shimmer's backstory... at least what I imagine of it. (I haven't read the comic backstory for her that will be in this month's Annual.) It makes sense to me that Celestia would be looking for a pony to wield the Element of Magic and help free her sister as the thousand year deadline approached, and the idea that she originally looked to the wrong one is compelling. Additionally, how much of Twilight's fears about disappointing Celestia could stem from following a much less-faithful student?
I know people made fun of Sunset Shimmer's "takeover Equestria with the mind-controlled population of a high school" plan, and it rightly does seem ridiculous, but I think it could have worked. The biggest assets she had under her control were her own versions of Celestia and Luna, who would (we can assume) become powerful Alicorns the moment they stepped through the portal. I could see Shimmer being able to transfer the knowledge of walking on all fours and casting spells to her thralls, so they wouldn't necessarily have to flail around like Twilight getting the hang of their new bodies. And so Shimmer wouldn't even need to "conquer" Equestria... Just get the existing Princesses out of the way and place her versions on the throne. If she could take the Crystal Palace by surprise and subterfuge and have a plan for removing the princesses, the rest of Equestria would never even know they had been conquered. And if push came to shove, she could always create herself a few more demon ponies... or even demon alicorns.
It's not the worst hand to play, by any means... and that's assuming she didn't have additional assets to her plan beyond what we knew about.