I'm not going to complain about realism in D&D specifically. I am going to ask, however:
Why do you believe that a fictional world will work as you expect it to work rather than possibly having rules that allow it to operate in ways antithetical to your perceptions?
What makes you think I expect it to work exactly the same? I expect it to work logically. The lava example is part of this. Under what conditions is magma/lava formed? Extreme heat and pressure. What happens to it when it gets outside a volcano and hits the air? It starts to cool. The heat is gone, the pressure is gone, it's going to eventually crust over. It might be flowing beneath the crust still but it's going to eventually all cool.
Now, you can say 'Well in my world rock melts at 300 degrees fahrenheit'. But that raises some serious issues. Cooking fires are easily going to get that hot. What are you building them on that isn't going to melt? What were ancient people likely to make things out of? Stone. Stone-lined ovens. We don't have metal yet so we're going to try to forge stuff on stone first- Whoops it melted. If you've got enough rock like that for there to be enough random lava flowing around in mountains(Which is bad enough of an assumption on its own, incidentally), it's going to be a fairly common rock. What becomes valuable isn't metal as much, it's rock that doesn't melt at such low temperatures.
Oh yeah, and the lava is still hot enough that simply being in the same enclosed room with it is going to be at best extremely unpleasant, and probably... You guessed it, deadly.
And for this we've created a fairly unbelievable world where the simple act of developing stuff became far harder, quite possibly to the point where civilization never really got started.
A floating rock in mid-air should be the result of a wizard in the normal world, not 'uh... Gravity is screwy here, yeah'. If you want fun with gravity, send the players to the Elemental Plane of Air, where down is where you decide it is and you can fall everywhere if you feel so inclined, or just not fall if you can convince yourself there's no down.
You keep the normal world relatively mundane, 'normal' except for the magic and the elves and what have you and whatever odd bit of actually interesting weirdness you have in it, and the players think 'well ok it all works like the normal world, except for the magic and the elves and what have you'. This isn't hard for them to accept or picture or get into. They don't know about the interesting weirdness or if they do they don't know the true nature of it.
And then when they find the area where the Far Plane is bleeding into the Prime and reality itself is going completely borked, that really grabs their attention from the very first thing that doesn't work the way it does in our world. They expect normal. They don't get it. This is immediately apparent to them, because the assumptions up to now have been normal. It's not 'oh another weird thing about this world', it's 'Merciful gods what in the nine hells is going ON here'. When they end up on the planes and things are *different* and you describe them, they notice immediately that 'hey, this is bizzare, never run into anything like this before, where are we?'.
And yeah, you can have interesting stuff that defies normal like, say, a mountain eight miles high that still has breathable air at the summit. But things like that should be used sparingly, and there should be a reasonable explanation for them. The world is flat, floats in a sphere a few thousand miles across, and the mountain is the centre point around which the world spins. You can breathe up there because you can breathe anywhere in the sphere. The stars are attached to the inside of the sphere, the sun-orb and the moon-orb are attached as well and move about along its surface. If you can fly long enough you can touch the stars. There is no lava, though, let alone cold lava, because the worldplate isn't thick enough to have magma chambers form and volcanoes occur. Other than being a flat disc, stuff is as normal. In fact, almost nobody knows about the world being flat. The world works normally otherwise, just like Earth. So when the PCs climb the mountain and see the edge of the worldplate, whoa. When they're sailing a ship through the dangerous waters said to be cursed because nobody sailing that way ever came back, and they find the edge and fall/don't fall off depending on what they do, whoa.
Stuff without even a good non-rational explanation like lava that doesn't really bother you takes away from stuff like that. It makes you think 'Uh... OK. That's kind of stupid' and you expect stuff to be off for no good reason. The lava encounter was on a totally mundane world that was apparently low-magic, where stuff worked normally otherwise. There was no reason for it, there were no special properties of the world, it was just bad random unbelievability. No wonder, no excitement, just 'wait what no.' Especially since the DM never told us... Pretty much anything at all about the world. >_>
THAT is what I'm complaining about. Random senseless purposeless shattering of the sense of reality that has nothing to do with how the world actually works and is.
"Wait if the world is flat and floating in a sphere how does it stay up? Maybe we should investigate and see if we can find out!" is great. "Wait isn't lava supposed to be really hot, why am I merely sweating, this is dumb" isn't. Next time you run into it make your DM explain it to you, and don't settle for 'Uh... Because that's how it works here'. If you're using it as a DM, give an explanation of why it works that way and why it hasn't screwed anything else up.
But yeah. Keep reality reasonably real so that when you screw with it for a reason the players are left going 'whoa'.
*shrug* YMMV. Me, I don't like random senseless non-reality and I refuse to accept its pointless unexplained use. Non-reality that actually has a purpose? Bring it. Have something corrupting the world, send me to the planes, make the world flat, or a cube, or whatever you like. Just don't have random pointless reasonless stuff that does nothing but pull me out of the game. Keep the fantastic sparse so it keeps its wonder. I've never seen unrealistic lava used to any purpose other than 'Wow, wouldn't it be dramatic if...'. Which, no. There's other ways to create drama that don't just make you sigh.
You know what would have made that puzzle I described somewhat believeable and not made me want to facepalm? Water. Simple ordinary water. The room is flooding with it. "If I don't get out of here I'm going to drown" is as valid a thought as "If I don't get out of here I'm going to be engulfed by lava", and doesn't have a "Wait, what" moment. It can even be reset easily! Trigger the drain, someone comes in and fixes the holes from the cannons, the reservoir behind the walls is re-filled and it's ready for the next pair of guys. Still deadly, but not as stupid.