Any level with a double fling.
Portal 2 has none.
Any level requiring timed portal placement. Some story segments when you're avoiding being crushed.
Portal 2 has none (except...two? in co-op mode and even then they're fairly forgiving).
Sometimes you have to correctly time reportals to keep yourself in a funnel
Fairly forgiving if you're not that accurate or fast. The funnel only needs to catch a small portion of the player's collision hull to grab them.
or accurately shoot the floor under a turret before it swisscheeses you.
Come on, turrets don't kill you that quickly.
And you shouldn't forget the boss battle, either. Snatching that last core was the most frustrating part of the whole thing.
More or less frustrating than Portal 1's boss battle? Likely less. GLaDOS's third core was 100 feet off the ground.
I feel like you're looking for a hardcore shooter or a hardcore puzzle game in a game that's meant to be a relatively accessible combination of the two. For most puzzle gamers many of the momentum redirecting puzzles (and potentially others, those are just the somewhat obvious ones) can take faster reflexes than they're used to needing and for most shooter gamers they have to think about what they're doing more and more often than they're used to. I can understand that you might think portal could be more fun as a harder (also more niche, less popular, less accessible, good for fewer people), but others disagree. It's all about personal taste, really.
Not really. What I am looking for is a progression in challenge. All Portal 2 had was "intro to this new mechanic," "combine new mechanic with a previous mechanic," and a few "moderate challenges with a mechanic." They were all, basically, intro puzzles. None of them got terribly advanced at all. No double-flings, no light-bridge redirection (beyond "stand here, move bridge so its still under your feet").
For heaven's sake, there were only two (
two!) puzzles in single player (and one in coop that I'm aware of) that required you to push yourself to the ceiling with the funnel in order to create a fling-drop. I felt so satisfied when I figured it out the first time (and then an idiot when I failed to figure it out the second time after nabbing a hint). Two more puzzles that used it would have rewarded me with the satisfaction of having remembered the trick after that.