No single definitive answer, but there are probably one or two key words that point to necessary basic ingredients. One such key word is "interest", which is multi-faceted and has far reaching implications. For example, an image can evoke personal interest, and that can take various forms such as emotional interest, intellectual interest, curiosity etc. On a less personal level we could add things like visual interest.

Images can evoke many things apart from interest, so on an even more basic level we could include "evocative" as another key word. There are no doubt various other ways an image can engage our attention (which points to another key word -"engaging").

this one pulls you into irs center, Nice shot
The title of the thread asks "What" as opposed to "In what way", so I suppose an answer requires specifics rather than general concepts. But the key words give us a starting point and specific areas to focus on. Apart from that, defining concepts like visual interest could be tricky, so I'll resort to the old saying "I know it when I see it".

I would rate the following image as one of the more engaging photos I've taken in recent years. Technically the foreground trees are a bit soft so it's not without its flaws, but the play of light on the mountainside makes it a very viewable image IMO, and that is where the viewer's attention will tend to gravitate. Because the focus of attention is off in the distance, the image has a noticeable 3D quality. Apart from that, a common quality that landscape photos often have is pleasantness.
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I've never been good at shooting indoors without a flash (or with a flash, for that matter!). We were exploring the old Vulture Mine ghost town, and we could go into all of the buildings. I was carrying my Canon 1DX (Yes, the one that Canon couldn't fix!). I put the ISO on auto - which is something I never do - and used AV mode. I liked how they turned out at the time, but then everything looks good on a 3" screen. But even on my 21" screen, I was pleasantly surprised. I did have to "fix" the perspective on some of them, but even with that, my normal develop treatment seemed to take care of most of them (highlights down, contrast and clarity up).

As always, your comments and suggestions for improvement are welcome!