Well first, integrated and "just works" are pretty high on the importance scale. Searching for PG clustering gets me a wiki with a matrix of different solutions, with apparently the main suggestion being DRBD. So just to start off, I've got to evaluate a number of third party products or start quizzing community members to figure out which is the most likely to survive or put me in a good position.

They might not be "hard to replicate" or unique, but having great options in-box, that are extremely simple to configure - that's worth a ton. If I was running a large-scale operation and SQL Server was costing me millions a year, things might be different.