Wow, if I'm understanding that correctly R's function arguments can have default values that depend on the values variables within the function have when the argument is first used. That seems insane!
If you look at attempts to do this stuff in python---e.g. patsy, which emulates R's formula DSL, and there's another project that emulates dplyr I don't recall the name of---you see they have to resort to parsing and eval'ing strings instead of working on expressions (language objects that represent ASTs), which is not nearly as nice or safe.
Edit: But just to emphasize your surprise -- yes, you can definitely be surprised by delayed evaluation in many contexts if you're used to more traditional languages.
> y <- 10
> wat <- function(x=10*y) { y = -y; x }
> wat()
[1] -1000
> y <- 10
> less_wat <- function(x=10*y) { force(x); y = -y; x }
> less_wat()
[1] 1000Welcome to R!