You know, I didn't overally hate EG, which is a minor disclaimer since I'm just gonna stick to my issues with the film here.
My biggest issue? Well, as others have said, I hated the character designs. The art style and well developed designs was what first drew me into the series. But with the exceptian of Rarity and Fluttershy, no one else felt like the characters we've been following for three seasons. I could never see Rainbow Dash and Applejack wearing skirts. It just made that element feel so lazy and quickly hashed together. They all felt too samey looking to me.
I also didn't like Sunset Shimmer. Not because she was a generic high school bully, but because she was just wasted potential. A former apprentice of Celestia? One whose been watching Twilight Sparkle all this time? One who has a strong magical knowledge about Equestria and feels jaded by her lack of alicornhood? Those are all great things to build an interesting villian off of! But nope, didn't happen. We just got the generic bad guy with generic (and rushed) reformation at the end. Lazy.
And finally, the plot. Now, don't get me wrong, I didn't dislike it because of the generic high school plot. But the fact I felt like I was watching the pilot all over again. The scene between Fluttershy and Twilight Sparkle meeting as humans? That didn't feel like a throwback to me, it felt blatant copy and paste. A lot of the beginning parts felt that way. The added twist of the Mane Five being former friends driven apart by Shimmer was a nice twist, but not enough for me to feel like I was watching something new.
So yeah, my basic problems boil down to wasted potential. They had so many good base concepts here and it just felt like they didn't develop them enough.
....god that's a giant wall o' text, probs to anyone who reads all that