I hate the style of story telling is opted by the anime/manga. Everything in slow motion, just before the climax, characters gets flash back to their past. Every time they went out of the wall, it was their "best chance and last hope of humanity" and everyone cheers for that, every time. But season 3 has something which no one expects and its also good! that is why s03 has such high ratings.. I literally dropped the series at s02 but came back just because of high ratings of s03. Season 04 was as good as its predecessor and final season seems to be going as good as s03(since I saw few reviews and MAL rating). You will not be disappointed is all I can say.
I cannot tell you what's different without spoiling it. Give s03 as shot.