The upside is that the company has excellent developer tools and infrastructure (I mean aside from AWS), very accessible principal engineers, a huge amount of paying customers so your work has real impact, and it seems like most teams have autonomy to work and deliver in their own way. Within a week or two of joining I was given ownership of an important internal product while also getting up to speed on the main event (the retail website). The technical challenges are there for the taking.

I also mentioned some of this at http://news.ycombinator.com/item?id=2488980