I personnally can't stand least time.
I play Portal to relax in an intelligent way, not to stess myself even more than at my work!
But least steps and least portals are perfectly fine to me, since you have all the time you want to think your next move. It is challenging, but not too stressful.
And about the achievements, I think this is really dumb, and even dangerous, to unlock features according to this. Things like the gamerscore on Xbox are concepts I greatly despise.
It gives even more advantage to "I play all the time because I have no life" players. Those guys are already advantaged by their experience in the game, and now you give them more stuff?
I can understand this in RPGs, because this collection aspect and "time means power up" are part of the genre since day one, but if all games become like that...
I fear for the social lives of a lot of people.
It is also a great waste of energy. All the time spent by those guys to get achievements even if it is not funny to them is effort lost for constructive and creative things (like, I dunno... mapping
).
Well, I am not saying anything when getting those achievements is fun to the person. But "forcing" people in trying to get them is sure plain wrong.
Fortunately, for now achievements on PC version of Portal are purely optionnal, which is perfect to me. You can just ignore the locks on the achievements you didn't get and move on.