I come from a Sybase SQL Anywhere shop. It never ceases to amaze me how stuff that can be elegantly expressed in a couple of queries in Watcom-SQL typically takes four times as much code in MySQL's dialect. I love Sybase's support for the ANSI standards, subqueries, Java/.NET/C/PHP/Perl stored procedures when they are the right tool for the job (ever needed to resize raster images in an INSERT trigger coming from some third-party application?), and great drivers. I shouldn't have to spend 10 minutes trying to figure out why MySQL doesn't support the standard casting string concatenation operator by default (||), or why subqueries don't work like they ought, etc.
Having used Postgres, all of the worthwhile MySQL features are there, most of the SQLA features are there, and the pain level is much, much lower in Postgres than MySQL for someone coming from a full-featured commercial RDBMS.
What really sucks is all of the applications that are so coded around MySQLisms that they don't run on ANSI-compliant engines.