I believe that relying on very basic and well-understood technologies at the architectural level forces you to save all your cleverness and new ideas for the actual app, where it can make a difference to users.

I think many developers (myself included) are easily seduced by new technology and are willing to burn a lot of time rigging it together just for the joy of tinkering. So nowadays we see a lot of fairly uninteresting web apps with very technically sweet implementations. In designing Pinboard, I tried to steer clear of this temptation by picking very familiar, vanilla tools wherever possible so I would have no excuse for architectural wank.

The other reason I like the approach is that the tried-and-true stuff is extensively debugged and documented. The chances of you finding a bug in MySQL or PHP as the author of a mid-sized website are microscopic. That's not the case for newer infrastructure like NoSQL or the various web frameworks.