I'd love to see an "R for rubyists/pythonists/...", basically an R tutorial aimed at reasonably experienced programmers which explains what you need to know about the quirks of R to properly grok it as a language, how it differs to ones you're familiar with, etc.

As opposed to a tutorial aimed at the "I really just want a recipe to achieve statistical task xyz" crowd.

In particular: every time I delve back into R, I forget how its funky data structures work.