Ok, I'll try to be a tad less angry when writing this and put it more bluntly.
Fairy Tail could have been good. But just wasn't. Early on, the story doesn't have much of a direction. Take Naruto for instance who wants to be Hokage, thus every journey we have is something that builds to him becoming Hokage. Natsu is different as his goal doesn't seem to built on. Finding Igneel doesn't seem like something that we get closer to through the arcs. This ends up making Natsu less interesting, and it robs Fairy Tail of direction.
Another poor point, is Mashima's poor planning(?) Or perhaps it's just straight up fan service. Probably a mixture of both. Characters, especially in the final episodes/chapters, are seen doing things they really should not be able to do. I won't spoil, but when a character does something that they have shown no prior skill at, and when there's other characters in the guild that can do the exact job infinitely better than this character can, it really just shows poor writing.
Even in 100 Years, things happen that are contradictory and don't make sense.
I'll put it this way, what if in Dragon Ball, Goku all of a sudden sprouted wings? Wouldn't make sense. Or what if at some point in Naruto, Kurama magically became able to destroy a universe. Wouldn't make sense. So Fairy Tail has little excuse as well.
There is also the problem of character deaths. Few people die. If you're expecting loss to push characters forward. Yeah don't expect it here. There are a few deaths, but nothing like some anime do.
Now, one thing this anime DOES do, is have some cool characters. Freed, Ignia, Mavis, Zeref, Mirajane, Mystogan, Rufus, Kagura etc.
These characters are actually great. And some of them, even more interesting than Natsu.
That being said, some characters that I could have respected, have lost respect due to how they were handled. Either from getting some random bullshit power boost, or from coming back from the dead when it would have served them better to stay dead.
I honestly liked Fairy Tail when I first watched it. But after looking back, and after watching better anime.
It's just bad.