Making the switch from MySQL to PostgreSQL?

Question

I've spent the last year becoming really comfortable with MySQL, but due to its increasing trendiness and my desire to homogenize my web-apps with Heroku, I'd like to start using PostgreSQL for my web apps instead. There are a resources out there for learning PostgreSQL, but I don't really want to have database concepts explained to me from scratch again, and I don't want to have to re-learn all the stuff that's pretty much the same.

What are the critical differences that I need to understand - syntactical and conceptual - between MySQL and PostgreSQL that will affect me on a day-to-day basis?