Cotton gins are obvious in retrospect - you basically strain the cotton through a mesh that's smaller than the cotton seeds and have an automatic brush that cleans the mesh to prevent jams. Put the straining process on a wheel instead of a straight belt and mess with some gears to make everything work off of turning one crank, and you're basically done. In retrospect it's straightforward, but we went centuries or millennia harvesting cotton without figuring that out.
Most things are obvious in retrospect - you can look at a traffic light and say "yeah, I'd obviously think of that - you need a bright and clear signal positioned where all can see it, and you need to have a warning light in addition to stop/go since cars have a braking distance" but it took years for someone to come up with it.
I agree that just because it's obvious doesn't mean it's not valuable, but my position is that it does not deserve the various government-granted legal protections of a patent. If it is obvious then it is something that you run into, not something that you create. By all means exploit your first-mover advantage as much as you can (be it a genre, a market, or a technique), but don't ask the government to grant you a time-limited monopoly. Just because it didn't exist for centuries doesn't mean it took centuries to create.
(note: "obvious", "straightforward" and "easy to understand" have various degrees of subjectivity attached so it's never easy to argument in these terms)
If it's obvious then it's not worthy of a patent, period.