Things that are likely to trip up new R programmers:

* Function arguments are always passed by value. Objects are copied if they are modified in a function.

* Function arguments are lazy evaluated.

* Watch out for automatic factor conversion when importing data. R will display your string data as text, but behind the scenes it will treat it as an integer.

* R is slow. Really, really slow. All your intensive calculations should be handled by libraries written in C, Fortran or some other compiled language. Your R code should be mostly for glueing things together.