[Des]

Sometimes it is just obvious which aspect should be used (see below); sometimes we have to work at it (which I am either not good at, or too lazy). Just for example, the first one is looking down the boardwalk/steps. The second one is the same boardwalk looking back.

Yes and sometimes you can see the same scene either way, just to emphasize different aspects.

This one directs attention to the cows walking back from milking.


Whereas this one puts the whole farm in context.


Vertical is a natural perspective for tall trees.


But a horizontal view shows the trees as part of a forest.

(This is an extreme example, because it is 8 vertical images stitched.)

There's an interesting thread which shows what can be done with landscapes in portrait orientation: Post Your Portrait-Orientation Landscapes! - PentaxForums.com

Computer monitors, and hosting sites like Flickr, are inherently biased against vertical shots. But they get a fair go on a tablet/phone you can rotate, and in a print.

[beachgardener]

I would like to see 16:9 frame lines when needed in the viewfinder or back lcd at least, don't use it much, but when needed...
Camera reviews don't say what sizes are available in camera, not sure if Pentax do 16:9?

[DeadJohn]

Got me wondering, am I alone in my overwhelming preference for landscape photos?

I prefer landscape but it's not an "overwhelming" preference. IMO, landscape orientation should be the default because it's how our vision and many of our display devices work. Most of my keepers are horizontal.

Portrait and square crops can be effective for some subjects. If you want to emphasize a vertical subject without surrounding distractions, portrait mode is the way to go. It works for tall buildings, full body portraits, and similar scenes. 2 examples follow.

[TaoMaas]

I usually try to match my camera orientation to my subject matter, depending upon what I'm trying to show. I think it's because I spent so many years shooting slide film and had to do my cropping in-camera.