I'll preface this by saying that I've been considering this question in light of the "Summer of Love" initiative and subsequent blog posts, which are looking at the extent to which comments are "friendly" on Stack Exchange.
Much of the impetus behind the movement is a commitment to treating new members better, and it's safe to assume that a majority of these new members are here for help in the process of learning something (e.g., programming, statistics, new software). Comments can both set the tone while also being a means through which to deliver learning.
The actual Stack Exchange study is measuring the response of a third party to the comments made on the site, but, as more readily available system, I'd like to find out if a teacher who projects a "friendly" attitude positively affects the learning process. Conversely, it's not ideal if a teacher uses "unorthodox" methods to motivate students (e.g., sarcasm), but controlling for all other factors, could these methods actually motivate some students to learn better?
While the analogy between educators, students, and Stack Exchange participants is far from perfect, is there support in the educational psychology literature for the notion that teachers reported by students as "friendlier" (on average) help those students achieve higher levels of performance on standardized tests and other evaluations?