Even as an intern, I felt like I could make a significant impact, largely because I was given nontrivial projects and the guidance and mentorship I needed to come up with good technical solutions. I also got to work on everything from front-end development to embedded systems and everything in between, and was given the freedom to implement a number of different features that I was interested in learning about.
The people were fun and brilliant, with a diverse set of skills, ages, personalities, and backgrounds.
The software release process was well-structured but not too restrictive, and it varied according to the subject area, with web development getting the most stringent regulation.
The internal tools were awesome. There was a lunch-ordering tool so we could get restaurant lunches delivered to the office, a "quips" tool to quickly add funny quotes said during the workday, and an extensive and well-documented set of tools for bug-tracking, version control, communication and announcements, server administration, a vastly improved way to grep on the servers, and a bunch of other dev-related ones I'm not thinking of.
The culture was really close-knit and social, due to lunch at the office and lunch outings, free food, soda, and beer that led to a lot of technical and nontechnical breakroom conversations, and frequent social gatherings.