I liked the film to death, but I too had a lot of critiques for it.
My main problem is that the plot is a rip-off of Twilight's Kingdom.
Basically, a ruler from a distant land (Tirek/Storm King) comes to Equestria to steal the magic of the Alicorns. By their side is a renegade from Equestria (Discord/Tempest) who is coerced into servitude through false promises but is eventually betrayed in the end, and ultimately reformed by Twilight.
The reason it failed in the movie was because the film managed to drag things out a lot longer whilst simultaneously cutting what could've helped the plot progress more, and much like any other film based on a TV show, it felt more like an extended episode of said show, and featured a road-trip among the characters.
My second problem is Tempest's redemption. Do not get me wrong, she is my favorite character in the movie and arguably in the Friendship is Magic franchise, but her redemption was pretty much the same cliche story of the villains backstabbing each other, and it confirmed her anti-moral of not relying on anyone. Finally, her redemption itself came across as shallow. Unlike Starlight, who redeemed her actions upon realizing the harm she could inflict as well as realizing how stupid her anti-moral was, or Trixie who was practically a victim of ostracization from society thanks to the Mane 6's unreasonable vilifying of her, Tempest redeems when the Storm King betrays her, and then Twilight refers to her as a "friend" when just the other day she tried to hunt her. Ultimately, it was the Storm King who redeemed Tempest by turning on her and not Tempest herself, which makes the scene of the Storm King's death look more like a revenge-plot rather than actual redemption. Again, the similarities between the film and the S4 finale really, REALLY hampered its quality..
My 3rd problem: The Storm King. Unlike most people, I found him to be an enjoyable comic relief villain, but that is the problem. He was introduced in the final act of the movie. If they were building up to him, maybe it would've worked, but they treat him like he was the antagonist all along when in actuality, Tempest did more as a villain. Seeing a villain this enjoyable to see but only at the backend of the film can be pretty bad.
Fourthly, some decisions in writing made the characters look... very questionable, especially our main protagonists. Apparently NO ONE in Equestria heard about the Storm King even though he had half of the world under his thumb. Cadence, Celestia, and Luna, despite being powerful rulers who have subdued enemies are useless against Tempest, Celestia apparently know Queen Novo and the Hippogriffs, enough to just drop everything and scream to them for help, yet when the Storm King trashed Mt. Aris, Novo didn't send help, and Celestia didn't notice their disappearance? The Mane 6 travel for a good 2 days to find an abandoned mountain, when they could've just gotten help from the Pillars, or the Changelings, Dragons, or even Twilight's brother. And I know the excuse is that the film was originally written in the same year that Season 4 ended, with the first 4 seasons in mind for the film's continuity, likely indicating that the film was meant to take place in between Seasons 4 & 5, but I think some decisions should've been changed when the movie was delayed.
Overall, the movie was flawed. Ok, but flawed.