Many solid points that hits home, to quote on of the points:

"Pretty standard. The reason why we chose each of these things was the same — reliability. Even memcached, which is the conceptually simplest of these technologies and used by so many other companies, had some REALLY nasty memory corruption bugs we had to deal with, so I shudder to think about using stuff that’s newer and more complicated. My only suggestion for choosing technology would be to pick lightweight things that are known to work and see a lot of use outside your company, or else be prepared to become the “primary contributor” to the project."

http://cloudspring.com/scaling-lessons-learned-at-dropbox-from-4k-to-40-million/