Some people prefer a difficult fast reflex way to get to the exit.
Some people prefer a complex thinking way to get the exit.
I think these really make what a map is. A puzzle is easy if you see the solution very fast and completing it is very simple. A puzzle is hard when you need high reflexes for it, and you need to think over it real hard to finish it. You can also add a time limit to the thinking time, such as in the trailer, to increase difficulty.
In maps there is also the results of failure, will you be killed, or will there be a fast way back to the beginning? Personally, I don't like being killed all day and I prefer falling down on a floor allowing you to climb back. Loading message annoys me most 
Dying in the game can bring a nice challenge though, and isn't bad at all if used right way.
Use of physic objects is a real fun thing to include in your maps aswell, I haven't seen anything such as a balance puzzle in any map yet. A map with lots of phyics is fun, but can also be very busy. Cubes are important part of the puzzles, but you can easily use other items aswell. In my maps I mostly place a radio around which you can take with you troughout the game if you like.
Building a small tower of blocks or objects, to gain height for a jump is a fun use of physics too.
Just to point out some things which could be loads of fun in maps.
I could be completely wrong about what I wrote above here, I don't have that much expierence mapping with portal at all.
The point of this thread is to tell people that there are loads of things not yet used in maps, exploiting these variations can live up your map and make it pop out of other maps.
