When you push something to the limit all technologies fail in their own special way. This lead them to evaluate tool choices with a preference for tools that are: mature; really good and simple; well known and liked; well supported; consistently good performers; failure free as possible; free. Using these criteria they selected: MySQL, Solr, Memcache, and Redis. Cassandra and Mongo were dropped.
This point is related to what I found to be the most interesting topic: sharding vs. clustering. Pinterest heavily favors sharding and I think they make a few good arguments to support their case.
You can watch a video of the talk on InfoQ.