[Myrmornis]

What has pandas borrowed from R, other than a 2D data structure with heterogeneously-typed columns?

I guess the data frame merge invocations are similar.

(I know patsy/statsmodels are introducing R's formula syntax to python, but that's not pandas.)

[hadley]

The split-apply-combine framework dealing with group by tasks (http://www.jstatsoft.org/v40/i01/paper, not that there aren't other precedents) for one. But generally, Wes has used R to figure out what people want to do, and then ported an elegant interface to python.