Hadley verse in general. dplyr/tidyr and piping %>% has made things so seemless that I would hate to lose them for the ability to run faster. I swear I have forgotten 75% of what I learned in base R because these libraries (Outside of data.table) just work so well with one another.

RStudio has also been a God send for me. RMarkdown and now R Notebooks just amaze me. Then I work in a very MS Office environment that ReportRs (http://davidgohel.github.io/ReporteRs/) is the most under valued library right now.

I didn't have these things 5 years ago. (RStudio was initially released Feb 2011 but didn't start with it till 2013.