I'm probably an outlier, but I have to say that the language itself is one of my favorite things about R.
Vector based, super powerful indexing of vectors, functional programming basics, lazy parameter evaluation, super convenient parameter matching and defaults, all these things make it super productive for me and let me deal with data far better than other languages. Matlab is similar in its ability to deal with data, but that's a language that feels far clunkier to me. Python has caught up with some of its packages, but it definitely feels bolted on instead of native to the language.