Difficulty rampup (and the rate at which you introduce new concepts) is very tricky to do right. Valve did a lot of user testing for Portal to get it as smooth

Difficulty rampup (and the rate at which you introduce new concepts) is very tricky to do right. Valve did a lot of user testing for Portal to get it as smooth as they did. If you're interested in this stuff, don't miss Portal's developer commentary.