However I felt that what let it down was that the grind was very noticeable. Apart from the environment, there was very little that was new on the final planet. It was the same mobs, weapons and strategies as the previous two worlds.

Compare Half-Life 2 (which is still one of my all time faves) which breaks up the gameplay and not only with the vehicle sections. How different was the feel of Ravenholm to the rest of the game? The section called Sandtraps (iirc) where you have to cross the sand by using the metal panels and gravity gun is unique. (And that is followed by a section where you control an army of antlions with the bugbait ball!) Then in Nova Prospekt where you have to defend that position using the sentry guns against the combine. Back to City 17 and suddenly, you’ve got a small squad of people following you. And finally, in the citadel, the game changes the way your gravity gun works and it’s all different again. Yet it can be argued that, in essence, the gameplay is repetitive and the “grind” is still there: each level/section is simply “shoot bad guys, solve puzzles, get to the end of the level.”

Likewise, you could, at a stretch, classify any shooter like this: Doom was “Run through levels, shoot baddies, find keys, finish level. Repeat.” Quake was “Run through levels, shoot baddies, find keys, finish level. Repeat.” Half-life was, well, I think you get the picture.