Pros
The people here are friendly and open, and generally humble despite the high bar we set for hiring engineers. While at other companies I had often gotten the feeling that asking questions or going over to talk to another engineer was an unappreciated intrusion on their time, people at Twitter welcome questions and collaboration, and are happy to help each other; engineers who have been here several years treat newbies and interns with as much respect as they would another senior engineer. The message I got when I joined the company almost 2 years ago was to "never assume people within the company know best." This is a mindset I fully support and admire Twitter for having - it is so easy and can be so crippling to a company and to your own personal development to get boxed into thinking "we know best" and dismiss challenging viewpoints and ideas from the outside world. I could say plenty more pros on other aspects of the company, but this is by far one of the most motivating reasons I came to work at Twitter.
Cons
We have a chronic problem with email volume. Pretty much everyone at the company suffers from a flooded inbox. We try to fix this problem by making lists for different types of emails so only people who want the emails will get them. Great idea in theory, but the truth of it is that people don't get on the lists they need to get on, which means people posting to the right list don't get the message to the people they need to, resulting in them just resorting to posting on the main list again.