Pros
In previous companies, I often had to fight for small things that I thought could make me more productive or the project more successful. For example, something as simple as requesting an ergonomic mouse was a hassle - with levels of approvals and complicated expense procedures. At Twitter, you just walk up to one of the IT offices and say "Can I have an Evoluent upright mouse?" and they just hand it to you without a single question. Everyone wants to do the right thing. If something is obviously a good idea, you're just expected to do it, you don't need to ask for permission. I've never experienced that before at other companies.
Cons
From a purely engineering perspective, there is a lot that could be improved, such as some engineering practices or code quality. I don't mind too much, because it means that there is still something to work on. There is a lot of work to be done to improve the engineering practices, but the good thing is that current engineering management are all focusing a lot on these issues, and things will improve!