Ask HN: Building a new web app, lots of content, what to use for database?

I expect the site-traffic will be fairly high, decent amount of db writes, medium amount of reads (most pages will end up cached). I have experience in SQL but not in setting up the infrastructure to build scalable SQL database architectures.

I am considering Redis and MySQL. Not too keen on Mongo or Couch and Cassandra seems overblown for my needs. Currently planning on using MySQL and figuring out the optimization stuff as necessary. What are the cost/benefits as you see them? If you were approaching a new project such as this what would you consider?