We've been using MySQL for MochiBot for a while. It worked OK, until this week.
MySQL replication is unreliable. We had two MySQL servers running, one as a master, and one as a slave; the simplest of all replication scenearios. The slave was used only for redundancy and load balancing. It receives only SELECT queries and never mutates data (none of the connections connecting to it even have permission to mutate!). What happened? Replication stopped -- but only on one table, with no errors. The binlogs are all synchronized, it just decided not to update that table anymore. The workaround for this was easy, we stopped using replication. Granted, I consulted a few people who are running really high profile and high traffic sites on MySQL and they've never seen this before, but I can no longer trust MySQL to replicate our data.
The final blow was MySQL's MyISAM tables are fast, but dangerous. What happens when you try and put a lot of data in them? They explode. There's an arcane MAX_TABLE_ROWS limit to a MyISAM table. If you hit this limit, it won't let you insert any more rows. It doesn't grow for you. Why not? Beats me. I should've known this, but frankly, I've never trusted MySQL to store an 8GB table before, so I never ran up against it. Never again!
We're through, and are in the process of migrating to PostgreSQL ASAP. The bonus is that we can use all the nice features of SQL that you lose when making the MySQL choice: subselects, views, triggers, stored procedures, etc. I really should've listened to myself and avoided MySQL in the first place.