Part of the joy of playing a fantasy game or a science fiction game (as opposed, I guess, to playing a real-life game where you work at an office or till a farm) is that, depending on the game, from time to time you'll be left with your characters staring at a vista that simply could not or does not exist in the everyday reality of the players. There's a certain danger to this; the more unreal the landscape, the harder it becomes to keep everyone grounded and imagining the same thing. This can lead to disconnection, video-game syndrome (Oh AWESOME an entire LAND of SMOKING CRATERS and STUFF!), and to a general inability to convey the scenery in a way that is meaningful and weighty.
There are a few ways to approach this. One is, of course, to ignore it. There are plenty of games were weird landscapes are the norm. There are plenty of DMs that can pull them off without sending players spiraling into a deadly verisimilitude drain. I don't think I'm one of them, and because the games I run tend to be more "realistic" (to resemble the real world in their details) I don't ignore this danger. I pay very close attention to it.
I approach it with a two-fold attitude. The first is that the blatantly unreal should be rare (at least in the 10th Age; when we're talking Spelljammer, it just comes with the property). The second is that it should be as palpable as possible and resemble something true as much as I can make it do so. My example here is the Boiling River of Elnuril, which has been poisoned by the presence of dragons. It is a large watercourse that now has the appearance of a geothermal spring. There is a stink of sulfur that accompanies it and steam wafts from its surface. However, I resisted the urge to make things like steam monsters since they seemed too video-gamey. A vampiric mist rising off the river doesn't make a whole lot of sense without a good deal of mental gymnastics.
However, it can also help ground the players for strange unrealities to have palpable effects. For example, spending too long in the sauna-like conditions near the Boiling River is bound to exhaust PCs as well as to leave them covered in a sheen of sweat and steam. Falling into the river is bad juju, of course, and even lingering too close to its banks can cause extreme heat fatigue.
Senses other than sight are good to engage when realing the unreal; sometimes, it takes a moment of dwelling on something that you might otherwise gloss over in order to really get across what being in the presence of it is like.
Sometimes, you don't have to spend that much time on it. Sometimes it's just ok without a lot of thought—yet I can't help but analyze these things and attempt to make a sort of logic out of them. So whether I am describing the strange sensation of flying through starfields in Spelljammer or witnessing the terrifying bodily glory of a dragon for the first time, I will often take extra attention to describe things that don't exist in reality with more detail than your every day wall or chair. I'm sure other people hit on this as well, longer ago and with much more alacrity and eloquence than I... but I offer it to you as a thought.