More importantly; if you've seen the "true horror of the undead" once, are you going to experience the same effect when exposed to it again?
Why not? "It's magic". And people who has seen (and killed) dozens of dragons, still have to roll vs dragon fear, in any edition I've played.
Rolemaster, in a variant rule for Fear effects introduced in Rolemaster Companion II, had a rule where PCs got a bonus to save vs fear based on the number of prior exposures to the same stimulus (eg undead, demons, etc). I can report from experience that the pain of tracking it probably outweighs any benefit of verisimilitude that it yields.
A different problem of Horrifying Visage is that the "push" is actually a target action taken immediately on the Wight's turn.
That works for me, but only as a "90%" case: That is, in 90% of cases the result of the visage will be that the target reacts to move away from the wight.
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But then, "Push" is misleading, in that it really isn't the same as a "Push" from a bull rush. In this case, "Push" is really "Forced Movement".
But then, what if you used Horrifying Visage against a held or webbed opponent? Or against a slowly oozing intelligent gelatinous cube which has a two square movement? What if the terrain behind the target was a slippery slope which required a climb or perhaps a balance check?
Your comment on Forced Movement is correct. In 4e the same mechanic (Forced Movement) is used for a range of purposes: to model physical pulling or pushing, fleeing in fear, closing rashly (Come and Get It), facilitating an ally's movement, etc. It's a very flexible, widely applicable mechanic. To work out what is actually happening in the fiction when it is used requires looking at other features of the context, the relevant power's keywords, etc. This is not very different, in principle, from the fact that hit point loss can mean a range of different things depending on context.
On the particular point about the held or webbed, 4e does have rules for this: forced movement breaks immobilisation but not restraint. So a grappled target of Horrific Visage will break free (his/her strength augmented by terror!) but a target restrained in webbing will struggle in vain.
The ooze is a corner case. Here is another, similar corner case: when an ooze or snake is knocked "prone", according to the rules it imposes a -2 penalty to ranged attacks from non-adjacent enemies. Why? (This penalty is very obviously meant to reflect the typical case, where a prone target presents a much smaller vertical profile than usual.) I think the average GM can probably adjudicate these corner cases without too much trouble (eg just ignore them, if the group doesn't care, or provide any modest tweak to the outcome that is needed to satisfy the group at the table).
The difficult terrain issue is covered fairly clearly by the rules: forced movement is not affected by difficult terrain, but is affected by blocking and challenging terrain unless the GM applies a special adjudication (for example, a push from a giant's club might send you flying through the air cartoon-style, so you don't fall down an adjacent pit but instead find yourself on the other side of it). So if you're fleeing from the wight and bump into a cliff, you're too panicked to climb! If you're fleeing from the wight and come across a pit, you might stumble into it in your panic!
The pit issue actually came up when I used a deathlock wight. The players (and their PCs) had worked out that there were pits around (I can't remember exactly how - they may have been in a lower-level room and seen the holes in the ceiling, and then gone back to the upper level and heading into the area which they could work out must have holes in its floor). So they roped together.
The room with the pits also had the wight, and it did blast their minds with its horrific visage. At least one PC did fall into the pit, but the rope held, and the dwarf fighter - who was the anchor for the roped-together party - made his STR check. The players were very pleased, because their foresight and planning had paid off!
When I run 4E I decide what the wight is doing and describe it. Based on that description, using blindfolds or averting your eyes may have an affect on its Horrific Visage.
What I find interesting about 4E is that it's not hard to resolve crazy actions that follow from the fiction
In my own wight encounter, it was the roping together as described above. (I can't remember now what the DC was for the STR check, but would have set it using the tables from the DMG.)