I tend to take the view of RDBMS works until it doesn't. Most cases that things like click data or event data that ends up taking billions of rows, in which case you aren't going to want it in a RDBMS anyways (you'd like to, but hah).
I'd like LevelDB on MySQL personally.
And then I see an presentation like this and think, "Yes, of course, you should have put that stuff in a relational database, what were you thinking".
Of course, most of the open source databases do sharding and replication and the like equally terribly, so it's not a perfect solution....