What was the point of this entire arc? A makeover?
I'd really a shame, it had some great potential at the beginning. We were back to the original duo, and they were given a chance to finally explore the world proper, while learning new things about the guild members and why they left like they did. Maybe some of them decided to do their own thing and never re-joined, others you'd never expect would join Lucy's group and become protagonists in their own right. Even the plot with Gray going evil, clichéd as it is, could've been good and really put the theme of friendship on the test for Natsu. But nope, everything had to be dealt with in a few chapters, none of the characters got a chance to shine because Natsu stole the spotlight, and after an astounding 20 chapters it's back to status quo.
I think the reason why I'm almost getting angry over this is because Mashima is breaking against some of the most important elements of storytelling: conflict, buildup and empathy.
Most of the problems introduced in this arc, for example Gray's 'betrayal' or the threat of Avatar, we're dealt with in the simplest ways that would avoid the most conflict. Gray wasn't really evil, he was just undercover. He was a jerk to Juvia, but really it doesn't matter because she forgives and loves him either way. Avatar? Psho, here's all the characters, let's deal with them in a few chapters and save the cool-looking ones for later. It makes it feel like the characters haven't grown at all. You need to introduce proper conflict that tears at the characters, so they can overcome it and learns something from it. It's impossible to feel anything for the characters, there's no tension for the story and battles - its just hollow. So when we eventually come around to a chapter that should've been shocking, like Zeref's, all that comes out of it was a "meh, so that's a thing now huh? What did Zeref even do up till now?". The only enjoyably parts were when the story slowed down and focused on more trivial, non-world-ending stuff and comedy.
Even Bleach and Naruto at least tried to hold on to the basic storytelling aspects at times (but have other things that drag them down instead). Here it felt like Mashima didn't even care, he just came up with a bunch of good concepts and then hoped the execution would fix itself.