This spell enables the user to cause a length of rope (6' to 24') to stand upright by itself, and when he (and up to three others) climbs to its summit, disappears into another dimension. The rope is simply tossed into the air and climbed. If undisturbed the rope remains in place for the duration of the spell, but it can be removed, and if it is the persons coming back from the other dimension will fall the distance they climbed to the top of the rope.The first thing to address is this concept of dimension. As I've said before, I prefer a "no planes" cosmology: there's one huge universe, but matter in the ethereal and astral states cannot be perceived by beings in the "real" state, and beings in these higher states of matter can pass through matter in the lower states. So, my interpretation of the way Rope Trick works is that the caster climbs the rope first, becomes ethereal, and creates a 30-foot diameter sphere of ethereality around the point of entry, sustained by the caster's presence.
There are some consequences to this interpretation. First, there's no "window", as some later descriptions of the spell state. The occupants of the ethereal region can see in any direction, subject to normal limits of vision. They are still in the physical location indicated by the top of the rope, just in a different state. They are invisible to those who cannot see ethereal matter.
Second, there are no hard boundaries to this area of ethereality. If the caster's colleagues try to explore the area around the top of the rope, they materialize when they pass beyond the edge of the spell's effect. This may mean being embedded in stone or falling from a height.
Third, the spell's ethereal effect is sustained by the caster, so when the caster leaves the area of effect, everything in it materializes. The caster can re-enter the ethereal state, re-creating the sphere of effect, but effectively the caster must be first in, last out.
The height of the ethereal region is based on the length of the rope. The spell mentions a 6-foot length of rope as the lower limit of the rope's length; this would be used to remove the risk of damage from a fall. Using a longer rope gives the opportunity of using the Rope Trick as a way to reach a ledge when a grappling hook won't work.
Under my interpretation, the spell can potentially be used to pass through physical obstacles. Say you know there's a room a short ways beyond a wall, but can't enter it. Use Rope Trick with a short rope, right next to the wall. People climb the rope, become ethereal, move 10 to 15 feet in the appropriate direction, and materialize on the other side of the wall.
I'll have to cover the topic of wandering monsters and the Rope Trick in a second post.