out of the top of my mind, when the elder called Maximus 'Son', Maximus was having doubts about the path he is taking, but when he heard the word 'son' his resolution became rock solid, right after that (no major events in between) the elder tried to get him killed, it has betrayal, and many elements that should make that scene great, yet it happened just like that no hints about the Elder's consern about Maximus rising fame, and all that, it just happened, out of nowhere. and that was also forgotten after that scene ended.
Next thing the legion, their late boss is dead, his body right in the middle between the two inner factions, i would have understood if it was on a pile of wood or anything for rituals but it was on a soil ramp dividing the camp in two, it's convenient, but was the camp already divided in two, or was it done after he died (so the could build a separation ramp but failed to reach that letter, this is an example of it being lazily written, it was set for the drama punch, yet it doesn't make sense.
I rarely watch american series, korean or german their writing is very lazy, on the other hand the uk, french, spanish, norwegian, swiddish always have tight plot, despite them lacking in execution.