I think the game is well worth buying at full price, it feels like a complete package. I forgot to mention the things I don't like about it:
- there are very few "true" boss fights, only 5 or 6, and 2 out of those you'll fight multiple times, namely the trolls and valkyries. Granted, they're somewhat different in terms of abilities, and the few fights you get are both brutal and epic.
- it's quite low key in the same way GoW 1 was, you only get to fight some of the lesser gods, it feels like they've saved up the more awesome bits for the following games. Don't get me wrong, the story is self-contained and satisfying on its own, but it ends on a cliffhanger that makes you feel as if all of this was just a taster when compared with what's to come.
- the out-of-combat movement is restrictive in the sense that you can't jump and can only climb/descend by pressing circle in specific spots.
Now, in terms of the changes from the first games and the supposed "mellowing" of Kratos.. I don't really see it. He was a very one dimensional character back then and here it feels like he has grown a lot, in a believable way. The rage is still there, but he has learned to control it, you can see it in the way he executes enemies, it oozes of maximum efficiency and getting them out of his way in a both satisfying and quick way without giving into his anger and basking into the carnage.
The interactions with the kid are well written and do not feel forced. At least at the start of the game Kratos is very rough and abrupt with him, and the emotional moments you would expect from a game such as TLoU are few and feel earned and natural. I think the acting is very good, from subtleties such as Kratos calling Atreus by his name in dangerous situations instead of "boy" or the softening of his facial features when he is worried about him. Also, I like the lessons he teaches the boy, it's all manly stuff and appropriate.
In terms of the world, it's not really "open". It's not comparable to HZD at all, I wouldn't say it has filler content aside from the collectibles, but some of them are well integrated into the game and go deep into the Norse lore. For example you find several shrines depicting myths and one of the companions explains them during boat rides. It's so well done that I felt sad once I had found all of them and there were no more tales to be told. The side quests are fun. In terms of content quality and "meaningfulness", I think it's much better than HZD. And yes, it's a slower game than the previous ones pacing wise, but I didn't particularly mind, there's a good mix of frenetic battles and quiet exploration and story development.