Once you got over the basics the problems weren't challenging enough
The fundamental problem with the difficulty of the problems was that the solutions were always linear. You need block X on trigger Y to enable action Z. This not to say that some of the linear problems weren't a challenge, it's more that the problems were limited to only a particular type of problem, which made the problems almost boring to solve after a while.
For example, there were no logic problem solving challenges. For example, imagine a 15 triggers and 5 blocks. By default, that gives over 360,000 combinations, how can you piece together what the right triggers are to use to cause some action to occur.
By making the problems only one style of problem to you create a game where all the solutions are attainable by everybody, but for people who want a more challenging environment, it comes up far too short.
After a while, I found the problems got to be boring. Funnels and goo where an interesting mechanic, but the problems involving them were uninspiring. Overall, I was disappointed.
EDIT: I see that 1 of 14 agreed with this review (I'm sure it will be even worse). I'm sorry, but I didn't find any of the puzzles mentally challenging. Perhaps they are challenging to you and as a result, you should give it a higher score. But I found each puzzle to be very linear. Do A then B, then C, then D. I was expecting more of a logic puzzle game, where it was Do A and B, then see if C or D should happen. But as such, there wasn't much to figuring it out. I played Portal (1) a ton with the expansions, but was expecting the puzzles to be significantly more complicated and more sophisticated. In a way, I found the puzzles easier than portal 1. Portal 2 is much more like an adventure game, but the game itself is too linear. Sorry. You might not like my review, but it's still uninspiring.
EDIT #2: It's currently 12 of 41 agreed with this review. Once again, the puzzles weren't hard enough. If you are disagreeing, then it means the puzzles were some kind of significant challenge to figure out. It would have been FAR BETTER if there was an option that would shut off all the hints that are every where. For example, I didn't need the little "X" on all the surfaces where I could place portals, or off coloured panels. It would have been great to have an option that said, turn off hints.
I've read the reviews, and everybody is giving it A++, but for me, there was no challenge. It was a lot of running around with maybe 2-3 hours of actual thinking that has zero replay ability. That's why it's a 4 and not a 9.