The biggest thing is probably it's lack of good debugging tools. Their tracebacks are not very informative. Their handling of arrays is significantly inferior to NumPy. For example, R has a tendency to have separate functions for applying functions along different dimensions of an array whereas numpy almost universally has uses an argument to specify along what axis to apply a function. Also doing much of anything non-statistics is a royal pain.
Whpearson recently mentioned that people in some other online communities frequently ask "what are you working on?". I personally love asking and answering this question. I made sure to ask it at the Seattle meetup. However, I don't often see it asked here in the comments, so I will ask it:
What are you working on?
Here are some guidelines