Rary the Traitor said:
- Do they allow the character(s) to move through walls and other solid objects? If not, can they seep through small openings or under doors, for instance?
Shadow walk involves travel through a completely separate plane of existence. Consequently, travel using this spell by nature ignores walls and other solid objects, unless such objects exist simultaneously on the Plane of Shadow and the Material Plane, which is possible according to the Manual of the Planes (MotP), but somewhat unusual.
Wind walk "alters the substance of the character's body to a cloudlike vapor," suggesting that the character is insubstantial and, as such, should be able to seep through small openings or under doors. IMHO, since vapors and other gases cannot pass directly through walls or other solid objects, a
wind walking character also would not be able to pass through such objects.
- Do they allow the character(s) to move through magical barriers such as a Wall of Force?
The spell description for WoF states that "dimension door, teleport, and similar effects can bypass the barrier," but that physical and ethereal creatures cannot unless the wall is not a complete sphere. The wall therefore would block a
wind-walking character, since that character still has a physical form (albeit altered) and is traveling from Point A to Point B.
IMHO, the wall does not block a
shadow walking creature. The wall states specifically that it blocks ethereal travel, but also states specifically that it does not block astral travel ("Teleportation is instantaneous travel through the Astral Plane", and the wall does not block teleportation). I therefore would be inclined to say that a
shadow walking creature could bypass a wall of force.
- Is a Wind Walking or Shadow Walking creature invisible? If so, can they be revealed by a True Seeing spell?
From the spell description: "Wind walkers are not invisible but rather appear misty and translucent. If fully clothed in white, they are 80% likely to be mistaken for clouds, fog, vapors, etc." So these characters are not invisible. Moreover, because
true seeing allows the subject to "see[] the true form of polymorphed, changed, or transmuted things," a TS spell would show the true form of a being altered by use of the
wind walk spell.
However, a
shadow walking creature is on another plane of existence entirely. The only extraplanar vision afforded by TS is the ability to see onto the Ethereal Plane.
Shadow walking characters are not on that plane, and thus remain both "invisible" and undetectable by the TS spell.
(Actually, I might be wrong here. TS allows the subject to "see[] through illusions," and
shadow walk technically is an "Illusion (Shadow)" spell, so one could rule that TS functions specifically to detect shadow-walking creatures. However, that creates a logical inconsistency with the inability of TS to spot creatures who are on the Plane of Shadow but are not using the SW spell.)
- What are the differences between the two spells (beside being divine and arcane magic...)?
See above. Basically,
wind walk transforms you into a fast-moving current of vapor, whereas
shadow walk transfers you to the Plane of Shadow and allows you to bypass conventional movement by traveling through that plane and re-emerging onto the Material Plane at a location of your choice.
- Can a Shadow Walking creature move in any direction in the Plane of Shadow, and reappear anywhere in the Material Plane? For instance, can a wizard cast Shadow Walk when he's at the bottom of a tower and reappear at the top of that tower?
According to the MotP, creatures on the Shadow Plane are still restricted to moving in two dimensions unless they can fly or levitate. So, unless the wizard can do this, he will be unable to disappear at the bottom of a vertical structure and appear at the top.
- Since Shadow Walk can be used to travel to other planes, can it be used to go to the Ethereal Plane (in 1d4 hours), move through a wall (assuming it can't be done in the Plane of Shadow), and come back immediately to the Material Plane? Or does the caster have to spend another 1d4 hours going back to the Plane of Shadow?
As I just said, moving through walls shouldn't be a problem on the Plane of Shadow. Since the travel to other planes is not very fully fleshed out, I'd say it's the DM's call whether travel back through the Plane of Shadow from the Ethereal would be required. The answer probably is yes, since in general, unless you're using a specific spell or item to travel to the Ethereal, you can't simply materialize onto the Material Plane from the Ethereal.
- What does the Plane of Shadow look like, anyway?
You want to check out MotP for this one. Basically, a dark, gloomy, and twisted reflection of the Material Plane, with places, beings, and ongoing events that may have existed only long in the past, in the future, or might never have been!
Thanks for your help. 
Welcome!
