I think the reason why many people didn't enjoy HLM2 as much was because they were expecting another HLM1. Hell, even I felt like something was off by the time I finished HLM2 for the first time and like it might have been a waste of money. But as I replayed it I got to enjoy the characters far more. I could see why
I started to enjoy the way the levels complimented the characters and how it forced me to change my playstyle depending on the character. I realized how the mechanics of the first game weren't even nearly as smooth, how the levels weren't even close to as interesting, fun and challenging as the sequels and the soundtrack still blew me away.
HLM1 is like an abstract acid trip of adrenaline. It leaves a lot of stuff up to the imagination of the player.
HLM2 is more focused on a story-telling experience. It wants to conclude the storyline and world of HLM and managed to shock me with its twists quite a lot.
HLM1 is played primarily for its gameplay.
HLM2 is played primarily for its storyline.
Once I accepted the difference I was able to enjoy both games on their own merits as two separate experiences.