I'm just about to rewatch cause I watched this ep for the first time in a crowd of people talking and livestream problems but first impressions: really liked it! Definitely a very different tone to previous openers, and I agree it might seem a bit subdued after the S4 finale epicness, but not necessarily a bad thing~
The whole creepy town/brainwashing aspect was really something new, and very creepy! I thought they built up the tension really well, and the M6's reactions - as others have mentioned, I really loved that Pinkie was the one to immediately pick up on something being off, which just goes to show she's not stupid - she knows real happiness when she sees it, because making ponies happy is what she's all about. And her expressions were great XD The contrast between her and Fluttershy was interesting in that way...it totally made sense to me that Fluttershy would be all "oh look how nice everypony is! :D", but I'm glad they didn't go the route of her being for realsies taken in by the cult and the others having to get her out. But you could see how, if she didn't have the rest of the M6 around her and was vulnerable, etc, she might have been, which I think was a good way of showing how the whole cult mentality works and the equal-ponies felt pressurised to fit in with whatever Starlight Glimmer was saying, for fear of being alone/everypony turning on them, and especially targeting less confident ponies (like Fluttershy would have seemed, and might have once been - we all know, though, she's a lot tougher than she looks). Creepy stuff D:
Speaking of, I loved Starlight Glimmer as a villain (and the song! idk if it's This Day Aria-quality just yet, but always appreciate a good villain song) - I like that she's so different from the previous season big bads, because she's just a regular pony, not a weird creature like Discord/Chrysalis/Tirek etc - and yet she has so much power over everypony in the village, and there were a couple of little moments that showed how scared they all were of her - she seems so lovely and sweet and ~just wants the best for everypony, but she could snap at any minute - also, I loved her interrupting Twilight's big speech XD The spray-on cutie mark reveal was awesome, and the whole thing with the staff not being special is a nice touch - so that means Starlight was just removing everypony's cutie marks herself? Cause that's gotta be some high-level magic...and did/does she have a plan for the wall of cutie marks, since she presumably doesn't actually believe in the equality she preaches and is just out for power/some other goal?? So many unanswered questions...I'm pretty sure she's coming back, since they surely can't just leave it with her running away like that (all other villains have met some more permanent fate), and I really, really hope we get to learn more about her and her motivations if/when she does.
I admit, I do sorta wish the M6 had gotten to do a bit more (besides Fluttershy - damn, I was so scared for her when Starlight was questioning her about who had betrayed them ;~; ) because I was a bit meh on the new characters...I can never remember their names except Party Favor, who I admit does give good meme face XD But for whatever reason, they didn't interest me that much - maybe they'll grow on me on rewatch? But it does make sense with the theme of the M6 ~spreading the magic of friendship~ and them learning that Starlight's way isn't the only way, it's great to be different and blah blah, so I understand why they did that.
Overall though, I loved the episode, can't wait to rewatch and bring on the rest of S5!!! ^___^