One thing that really sticks out - there's a LOT of MySQL on here. and very little Mongo and Riak. That's not to say those are not useful technologies but it's clear SQL Scales.
Not disagreeing with you -- there have obviously been sites like Slashdot and Reddit that scaled on MySQL for a long time before the current crop of NoSQL solutions, but it could also indicate that scaling MySQL is non-trivial and hence, worthy of noting tried and true solutions to emulate.
It's worth noting that the current crop of NoSQL databases isn't _that_ old, and many of these sites were designed and built years before mongo or riak were really production ready.
I'd bet that in 3-5 years this list will have more nontraditional databases on it.
