Fairy Tail is one of the most popular anime series ever. The last chapter of the manga was pretty much lame without any good fighting sequence. Therefore, the addition of Mavis and Zeref was unnecessary which means that it would have been better without them too. There were many weak moments in the final chapter which ruined everything honestly.
The ending of the series is left open by Hiro Mashima due to the fact that another sequel is on the way which is 100 years quest arc which we have high hopes for. The only thing I liked about the show is its openness which is also associated with Fairy Tail somehow in the end.
Fairy Tail is all about taking quests or going on various quests mainly which is the main highlight of the show and this is also the reason that people like this show.
Coming back to main question ending chapter of the manga. First of all the author shows us about all the power ups that various character had gone through in the past one year. But, to the disappointment they ended up struggling in the face of Alvarez Empire. So, what was the whole power up for.
Many of the fights which happen in the series is all about friendship is the power in a battle and as well as all the battles are very short as they ended very quickly which was a let down. Their were some of the characters which we thought would be dead after fighting with natsu like Erza and gray but they all survive through this which doesn't make any sense.
The last battle between Natsu and Acnologia is one of the biggest let down that I had seen in the recent times. It doesn't have a proper battle sequence. It just seems random colors are popping out of Natsu's hand when he use different dragon slayer's power. It was certainly a letdown as their seems no effort from the other characters in term of fighting witj Acnologia.
But, they all handed over the task to Natsu it just seems like that and this is the thing which I hate the most as without Natsu no dragon slayer is able to land a hit on Acnologia. Their power up was just for show off. So, yes if I had to describe Fairy Tail ending than it is certainly a let down. I hope they don't do this thing again with the 100 year quest.
Why people hate Fairy Tail?
There are many reasons for hating fairy tail too first of all which is complete rip off of characters from other anime series. Not just that but the author goes straight to copy their powers too. It just feels like that author just doesn't believe in any original content and straight away copied the characters.
Fairy Tail is known for its Fan Service the most. Most of the viewers watch Fairy Tail anime just to enjoy the Fan service. Sometimes we can see Fan service in a very irrelevant place for example in a fight suddenly author place very exotic scenes which is just mindless placing of Fan service just for the audience engagement. It genuinely feels just like that.
For the information it is a shounen series and not some echhi series. Why does Fairy Tail focus on fan service instead of spending time on improving the plot line and story of the manga. If it was an Echhi manga than also it would have been ok in showing these kind of things. As well as the targeted audience of this series is ranged between 10-20 years old. Like Bleach, One piece they are also a popular series but they don't focus on these things. On the other hand their main focus is on story line.
It was ok when the concept was first introduced but after that it started to get misused as author placed Fansevice wherever he liked. So, this is also a reason why people dislike Fairy Tail.
There is also a problem with the battle sequences in this manga and on the other hand this manga says to be a fighting action manga which doesn't looks like the case here. Sometimes it feels like that Fairy tale battles is only based on Friendship. As if you have stronger friendship than you will have stronger powers and you will win tye fight easily with the friendship power which is totally senseless.
For example if the main protagonist of the story is fighting one antagonist and he is losing the fight and suddenly his friend will pop up which will eventually lose to and will say something emotional which will ignite the powers in main protagonist amd suddenly he will become very powerful and will eventually win that fight which is literally very dramatic.
This kind of tricks can work for one arc or two arcs maximum but literally repeating the same sequence makes the fighting sequence eventually very boring to watch. As we don't have the tendency to watch the same thing happening again and again.
And to add to this point there is also a lack of character development that we can see from the starting of chapter 1 to till now the latest chapter. There is very less focus which is given on character development. For example Natsu learned some lessons from Gildarts which he completely discarded in his fight with Zancrow. So, the main storyline is very weak.
Take Erza too in this point at the very starting of the series she is shown to be very powerful and smart but as the time passes on and as the events started to unfold she became the source of only Fan service as well as her character was made somewhat dumb which is very bad idea by the author. Overall, there are many weak points in the Fairy tail which makes people to hate it.