I find low detail so much easier to play in than high detail. It's a lot easier in low detail to focus on the visual aspects that matter because anything else that is redundant is not present. For example, I find Dg'ing in low detail a lot easier because keys and drops etc. are easier to spot.
The case for low detail is (also) clearly a matter of efficiency then, since it serves as a filter to remove everything that is not necessary/not important. Although I don't care about efficiency (anymore), but just about the gaming experience, for which I strongly prefer low detail.
I see what you mean about the DG keys, they're so hard to spot on high detail
http://i.imgur.com/w2MKztJ.png
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Visual fidelity is a huge part of games for me, and I hate it when people complain that you shouldn't care about graphics because "the gameplay is important", how are graphics not a part of the game play? - My guess is you have not played much outside of RuneScape, at least nothing graphically intensive, you start noticing the difference detail makes a whole lot quicker.
I think the problem with RuneScape is the fact you are never immersed in it, you're always doing something on the side so it doesn't really matter what it looks like. Or maybe that's just me.
Meanwhile, compare that to something like Tomb Raider, GTA, etc, where I found myself easily lost in it for a few hours, graphics matter a lot more for that immersion aspect.
Edit: And yeah, if they removed HD entirely I'd probably stop playing too, I could not play something that looks that bad.