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To answer the maria question:
Yes it is a drop in replacement.
However, the bulk of the development is still going to be done by mysql/oracle. Even they have admitted that. It has some cool improvements, xtradb over innodb, aria over myisam. Switching to use them may make sense in some situations, but many people
Obviously, postgres is not a drop in replacement for a non trivial app that uses mysql. At this point, despite Oracle's crummy behavior, there isn't a legitimate reason for such a huge change. Its still open source. Its still working. Nothing Oracle can do will stop it from continuing to work as well as it does. At some point if they stop development on it, or postgres reaches a level of significantly higher performance/features* that your app uses, then yeah, switch then.
Note, many application use mysql's built in replication which works very easily and very well, and is very mature. Postgres just released what they consider to be an answer to mysql's replication in postgres 9.0. I haven't had time to look at it, but its the first version of it. I wouldn't advise anyone to be the first to try it for a critical situation, out of sheer pragmatism.