While power hungry villains are common, I found Tirek to be a clever, intimidating, and suitably venomous character, who more than held up his part of the story. And what he "did" was drain everyone's power, leaving them tired and crippled, strain/break friendships, and take total control of the ponies' world entirely. I don't see how testing his power on some trees is a point against him. Not that the designs of rainbow power and the castle weren't weak, but those are offhand sights, not significant plot points.
Starlight Glimmer's face turn also makes more sense than the baby in Son of the Mask. That's still doesn't make it any better. (Actually, the more I think about Starlight's redemption, the more I start to wonder if I was really way too easy on it.)
I'm not quite sure I understand your point with Discord (who I believe most fans agreed usually made some of the best episodes, even though his last one was botched by the newbie writer who wrote Princess Spike), but I think it's that he has less potential than Starlight Glimmer. I'd have to say Discord has proven himself to be a good character and however well he does is up to the writers now. As for Starlight, I actually thought she was a good villain in her first appearance. She had potential as a villain, and I agree with you that she has potential as a hero, but that won't make me more positive towards her very weak redemption story.
Anyway, on my card, this would probably have to be the 2nd best Josh Haber episode. I'd say 8/10 or maybe 7.5