Derpy's canonization (having started with shirts and promos, with it all but confirmed in "The Last Roundup") is one massive catering to the Brony community.
A number of seemingly minor interactions by the background ponies are a result of Ascended Fanon, such as Lyra Heartstrings and Bon Bon hanging out together, Berry Punch continuing to be an implied alcoholic, and Derpy having a muffin pin on her carry bag.
Oddly enough, the animator for "Call of the Cutie" confirms that they were originally trying for that interpretation of Berry Punch, but the belch that would originally have followed her drinking from the punch bowl didn't make it past Executive Meddling.
The mention that Alicorn Twilight and the possibility of a human-based adaptation being introduced has sent some bronies into an uproar, however. One of the main reasons of their complaints, of course, is this, due to the fact that there's a metric ton of fanart done of these very things well before they officially existed.
As described as above, some view the pandering as a negative trait. It's sometimes viewed as exploitation, a crutch to allow the writers to slack off in favour of cheap tricks to get laughs and cheers from the fanbase. It's impossible to please everyone, as several bronies absolutely cringed whenever they found Ascended Fanon.
Periphery Demographic: The very existence of this work page's Troper Critical Mass and its many Trope Overdosed subpagesnote Even those subpages have subpages of their own. is evidence enough. You can go to any website and you're guaranteed to discover a member or two who is a fan of the show or even the existence of an entire brony subcommunity within that site. The fandom has grown so large that even news media is taking notice.
Notice the number of reviews on this site.
Scapegoat Creator: It used to be Dave Polsky, but since then Merriwether Williams has become the prime target for fandom scorn. Unfortunately her first aired episode was not the best hoof to start out on, and since then, arguments have mutated from her just not getting the show, to more than a few fans pinning everything they dislike about season 2 on Merriwether's shoulders. Merely mentioning her name on many fan sites can cause an immediate Flame War. Never mind that she's just the writer, and has to go through editors and other members of the production team before her episodes are aired, or that she's helped write great episodes on other shows before FiM *cough Band Geeks cough*. No, she's just a bad writer that doesn't "get the show."
This hits incredible levels of Fridge Logic when you consider that her next episode "Hearth's Warming Eve" was extremely well received by a majority of the fanbase, and her episode "Wonderbolts Academy" was considered one of the high points of season 3.
It got even more ridiculous with season 4's "Bats!" Initial fan reaction to the episode was positive, but once word began circulating that Meghan McCarthy's writing credit was in error and it was actually Williams who wrote the episode, suddenly some fans didn't like it so much (though plenty of people still liked the episode, and there were some people who didn't like the episode even before this was revealed).
So Okay, It's Average: Consensus on Season 6. It's generally seen as a "meh" season, since a lot of its episodes were criticized for being less interesting and memorable than average. Still, it's not exactly considered terrible, since it did have a few widely-liked episodes.
The lead up to the "Royal Wedding" made many fans angry over just about everything. Of course, most fans agreed it was much better than expected.
"Magical Mystery Cure", on the other hand, was incredibly divisive before airing due to its use of Nothing Is the Same Anymore, and remains fairly divisive to this day, although not entirely because of Princess Twilight.
Some Slice of Life fans who were drawn to the show by season 1 also feels this way about season 2.
The pony names from the toys (which in most cases only JossedFan Nicknames rather than changing previously-established official names). To say that the fans weren't happy is understating things.
Some Japanese fans are upset at the fact that the Japanese version will be using different opening and ending themes, preferring that they use the original theme songs. One fan even made a "Hitler Reacts" video in frustration.
Tough Act to Follow: After Seasons 4 and 5 being seen by many as the two best seasons of the show, Season 6 was hit with this. While still considered a decent season by the fandom, it was criticized for the overarching plot of Starlight Glimmer's redemption, not having enough standout episodes and a lot of the episodes suffering from They Copied It, So It Sucks.
Scenes that deserve special mention are Rainbow Dash's Sonic Rainboom, Princess Cadence and Shining Armor's love spell, the underground cave Queen Chrysalis traps Twilight in, the pegasus-powered water tornado in "Hurricane Fluttershy", Twilight's transformation sequence in "Magical Mystery Cure", Luna's transformation into Nightmare Moon in "Princess Twilight Sparkle" and the shot of the Cutie Mark Crusaders running against a sunset backdrop in "Flight to the Finish".
There are also many instances where the possibilities of Flash are used very cleverly. For instance, populating backgrounds with more or less randomly-generated ponies rather than using Faceless Masses.
Not to mention that these background ponies aren't just standing motionless in the background either. Looking closely in the background one can see them blinking, shifting and moving slightly, and talking to eachother.
And if what we have seen of season 4 so far is any indication, the animation quality is set to just keep improving. A particularly noticable change between seasons is that now, when there are close shots of characters, the backgrounds actually have depth of field!. We had seen depth of field tricks occasionally beforehand (Usually shifting depth of field from foreground to background), but now it seems to have become the norm for closeup shots.
With the Season 4 Finale, let's just say it's obvious where a fair chunk of Season 4's budget went. The effects in these two episodes are some of the greatest yet seen.
Game Informer took a very lighthearted and harmless jab at the show in their Timeline feature, which led to an [oversensitive brony] sending an angry letter ranting about how the ponies are badass and "not frilly". They poked some lighthearted fun at him while sincerely acknowledging the show's quality... [which led to more oversensitive bronies sending hate mail. By then, they could've easily ripped the bronies a new one and no one would've blamed them — yet incredibly, they took the high ground and vowed not hold a grudge against the fanbase. Instead of publishing any of their hate mail, they published a couple of letters from levelheaded bronies apologizing for the idiots in the fanbase. GI also specifically stated that frilly does not equal bad or lame — a sentiment Lauren Faust would agree with 100%.
Within the fandom, there are many people who voice their negative opinions of the show or certain episodes, and they are very loud and obnoxious about voicing their hatred. But judging by the amount of thumbs up comments praising the show or calling out the haters on EQD get, we have a Silent Majority of sane bronies.
Another vocal minority which evolved as of late are doing the opposite; being loud and obnoxious about anyone daring to talk against any aspect of the show, regardless of the arguments used. Their love of the series is such that any criticism is answered by sheer hatred and prejudice, which said Silent Majority only rarely acknowledges. Especially prevalent on EQD, in which repeat posters routinely insult and despise anyone going against the majority.
After the comparatively lukewarm reception of season 3, season 4 was largely heralded as a welcome return to form, as despite its own share of less popular episodes, it featured an unprecedented level of forward planning and nods to the fans.
After the divisive reactions to Season 6, Season 7 saw popular writer duo Kristine Songco and Joanna Lewis take over as story editors. Reception during their tenure has been very positive; many fans have felt the show has gathered a better input of new writers this time around, with the season including several worldbuilding and backstory elements that many viewers have been hoping to see for years and a strong focus on the season's theme of family.