WITH and window functions are portable (and both awesome - they will change the way you write your SQL for the better). I'm not sure if WITH RECURSIVE is portable - my recollection is that oracle and db2 at least don't need/use the RECURSIVE qualifier for recursive ctes: they just work it out for you.

Time intervals are also in the SQL standard as I recall (although not in the format listed here - postgres supports standard syntax too though). Not sure how fully supported it is cross-database, though.