"It gains the ability to fly at one half its normal speed if not under the effect of the gravity. It can hover, but it is a clumsy flier."
That's pretty much it. When characters are on the astral plane, they gain the ability to fly at half their normal movement rate (unless they can already fly). Pretty simple and to the point.
Here is the equivalent rule snip from the 1st Edition Manual of the Planes on moving in the Astral Plane:
"Mental movement is achieved by willing oneself in a direction. The maximum speed possible by this method is 10 yards per minute (30 feet per melee round) per intelligence point...Encumbrance slows down the astral traveler by 10' per round for every 10 lbs (100 gp) carried. Intelligence determines additional carrying capacity (use the Strength table on page 9 of the Player's Handbook). Magical items (but not normal items under an enchantment spell) have no encumbrance."*
*NOTE: I left out the information about physical movement versus mental movement, etc. I think that even the excerpt above more than proves my point.
Good grief. How clunky is that?! It seems that not all rules have gotten more complex over time...